Cleanup

- some code reorg
- rename all internal functions to use `bmfs_` prefix
- update write command to create file if it doesn’t already exist

Author: Ian

doc/BareMetal File System.md

@@ -27,9 +27,9 @@ The first and last disk blocks are reserved for file system usage. All other dis
 
 	Block 0:
 	4KiB - Legacy MBR (Master Boot Sector) sector (512B)
-		 - Free space (512B)
-		 - BMFS marker (512B)
-		 - Free space (2560B)
+		- Free space (512B)
+		- BMFS marker (512B)
+		- Free space (2560B)
 	4KiB - Directory (Max 64 files, 64-bytes for each record)
 	The remaining space in Block 0 is free to use.