[BOOTSECT][FREELDR] Rewrite the Ext bootsector (reactos#7544)

Rewrite the Ext bootsector because the older one was broken and had no compatibility with Ext4 extents.
Introduce a 3rd-stage bootsector for complex code.

CORE-14235

- Why was the previous bootsector broken?
  Because of hardcoded inode size, hardcoded freeldr base address, etc.

- Why is there a extldr.sys?
  The extldr.sys was introduced because the limited code space in the bootsector
  prevents adding new features, such as Ext4 full extents support.

- What is extldr.sys and What does it do?
  It is the helper file for the Ext bootsector and that is necessary for adding
  Ext4 support. It locates the freeldr.sys file, loads it into memory and runs it.

Author: iLauncherDev

boot/freeldr/bootsect/CMakeLists.txt

@@ -4,7 +4,8 @@ include_directories(${REACTOS_SOURCE_DIR}/boot/freeldr)
 if(ARCH STREQUAL "i386" OR ARCH STREQUAL "amd64")
 
     CreateBootSectorTarget(dosmbr ${CMAKE_CURRENT_SOURCE_DIR}/dosmbr.S ${CMAKE_CURRENT_BINARY_DIR}/dosmbr.bin 7c00)
-    CreateBootSectorTarget(ext2 ${CMAKE_CURRENT_SOURCE_DIR}/ext2.S ${CMAKE_CURRENT_BINARY_DIR}/ext2.bin 0)
+    CreateBootSectorTarget(ext ${CMAKE_CURRENT_SOURCE_DIR}/ext.S ${CMAKE_CURRENT_BINARY_DIR}/ext.bin 7c00)
+    CreateBootSectorTarget(extldr ${CMAKE_CURRENT_SOURCE_DIR}/extldr.S ${CMAKE_CURRENT_BINARY_DIR}/extldr.sys 8000)
 
     CreateBootSectorTarget(fat ${CMAKE_CURRENT_SOURCE_DIR}/fat.S ${CMAKE_CURRENT_BINARY_DIR}/fat.bin 7c00)
     CreateBootSectorTarget(fat32 ${CMAKE_CURRENT_SOURCE_DIR}/fat32.S ${CMAKE_CURRENT_BINARY_DIR}/fat32.bin 7c00)
@@ -31,7 +32,8 @@ if(ARCH STREQUAL "i386" OR ARCH STREQUAL "amd64")
     CreateBootSectorTarget(isombr ${CMAKE_CURRENT_SOURCE_DIR}/isombr.S ${CMAKE_CURRENT_BINARY_DIR}/isombr.bin 7000)
 
     add_cd_file(TARGET dosmbr DESTINATION loader NO_CAB FILE ${CMAKE_CURRENT_BINARY_DIR}/dosmbr.bin FOR bootcd regtest)
-    add_cd_file(TARGET ext2 DESTINATION loader NO_CAB FILE ${CMAKE_CURRENT_BINARY_DIR}/ext2.bin FOR bootcd regtest)
+    add_cd_file(TARGET ext DESTINATION loader NO_CAB FILE ${CMAKE_CURRENT_BINARY_DIR}/ext.bin FOR bootcd regtest)
+    add_cd_file(TARGET extldr DESTINATION loader NO_CAB FILE ${CMAKE_CURRENT_BINARY_DIR}/extldr.sys FOR bootcd regtest)
     add_cd_file(TARGET btrfsvbr DESTINATION loader NO_CAB FILE ${CMAKE_CURRENT_BINARY_DIR}/btrfs.bin FOR bootcd regtest)
     add_cd_file(TARGET fat DESTINATION loader NO_CAB FILE ${CMAKE_CURRENT_BINARY_DIR}/fat.bin FOR bootcd regtest)
     add_cd_file(TARGET fat32 DESTINATION loader NO_CAB FILE ${CMAKE_CURRENT_BINARY_DIR}/fat32.bin FOR bootcd regtest)