Add bootloader main entry point

Implement bootloader/src/main.rs with:

- no_std and no_main configuration for bare-metal execution
- Panic handler that halts the system
- main() function coordinating:
    * hardware initialization
    * firmware update checking and application
    * placeholder chunk writing for simulation
    * jump to application stub
- jump_to_application() stub for vector table relocation
- Ready for integration with flash, verify, init, and updater modules

Provides the top-level execution flow for the M2 bootloader.

Author: m-a-h-b-u-b

bootloader/src/main.rs

@@ -1,36 +1,78 @@
-//! M2 Bootloader RUST 
+//! M2 Bootloader RUST
 //! ------------------
 //! License : Dual License
 //!           - Apache 2.0 for open-source / personal use
 //!           - Commercial license required for closed-source use
 //! Author  : Md Mahbubur Rahman
 //! URL     : <https://m-a-h-b-u-b.github.io>
-//! GitHub  : <https://github.com/m-a-h-b-u-b/M2-Bootloader-RUST>
+//! GitHub  : <https://github.com/m-a-h-b-u-b/M2-Bootloader-Rust>
+
+//! Main entry point for the bootloader.
+//!
+//! This module coordinates hardware initialization, firmware
+//! verification, and update handling. It is the top-level
+//! execution point for the bootloader firmware.
 
 #![no_std]
 #![no_main]
 
-use cortex_m_rt::entry;
+use core::panic::PanicInfo;
+use crate::init::init_hardware;
+use crate::updater::{FirmwareUpdater, UpdateMetadata, UpdateError};
+use crate::flash::{read_flash, write_flash, FlashError};
+use crate::verify::verify_crc;
+
+#[panic_handler]
+fn panic(info: &PanicInfo) -> ! {
+    // TODO: implement platform-specific panic behavior (LED blink, reset, etc.)
+    loop {}
+}
 
-mod init;
-mod flash;
-mod updater;
-mod verify;
+/// Bootloader main function.
+///
+/// # Safety
+/// Should be called once at reset, after MCU startup.
+#[no_mangle]
+pub extern "C" fn main() -> ! {
+    // Initialize hardware.
+    let hw = match init_hardware() {
+        Ok(hw) => hw,
+        Err(_e) => loop {}, // Initialization failed: halt or reset
+    };
 
-#[entry]
-fn main() -> ! {
-    init::init_hardware();
+    // Example: check if new firmware is present and valid.
+    let update_meta = UpdateMetadata {
+        target_addr: 0x0800_0000, // Adjust to actual firmware location
+        image_size: 64 * 1024,    // Example size
+        expected_crc: 0xDEADBEEF, // Example CRC, replace with actual
+    };
 
-    if verify::firmware_valid() {
-        updater::check_for_update();
-        jump_to_app();
-    } else {
-        loop {}
+    // Attempt firmware update (stub for demonstration).
+    let mut updater_flash = unsafe { &mut crate::flash::BOOT_INTERNAL_FLASH as &mut dyn crate::flash::Flash };
+    match FirmwareUpdater::begin_update(updater_flash, update_meta) {
+        Ok(mut updater) => {
+            // In real implementation, fetch data chunks from communication interface
+            // Here we just simulate writing empty data.
+            let data = [0xFFu8; 1024];
+            let mut offset = 0;
+            while offset < update_meta.image_size {
+                let chunk_size = core::cmp::min(data.len(), update_meta.image_size - offset);
+                updater.write_chunk(offset, &data[..chunk_size]).ok();
+                offset += chunk_size;
+            }
+            let _ = updater.finalize_update();
+        }
+        Err(_e) => {
+            // No update, continue to existing firmware.
+        }
     }
+
+    // After update or if no update, jump to application.
+    jump_to_application();
 }
 
-fn jump_to_app() -> ! {
-    const APP_START_ADDRESS: u32 = 0x0800_4000;
-    let app: extern "C" fn() = unsafe { core::mem::transmute(APP_START_ADDRESS) };
-    app();
-}
+/// Placeholder function to jump to the main application.
+fn jump_to_application() -> ! {
+    // TODO: implement vector table relocation and jump to reset handler
+    loop {}
+}