multiple changes: BL1/BL2/kernel separation (build system, etc.) BL2 implementation. BL documentation

bootloader/README.md

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+# ClassicOS 2-stage bootloader
+
+Bootloader documentation for ClassicOS
+
+## Disk image organization:
+
+```
+[  512 B  ] [      2048 B      ] [    Unspecified    ]
+  Stage 1         Stage 2               Kernel
+```
+
+## Stage 1 (`stage1.asm`)
+
+Responsible for loading the second stage using BIOS routines, and switching to protected mode.
+
+- Queries CHS parameters from BIOS
+- Loads the second stage bootloader (2048 B) to `07c00h`
+- Sets up a GDT with descriptor entries for code and data both covering the whole 32-bit address space
+- Enables A20
+- Set CR0.PE (enable protected mode) and jump to stage 2
+
+## Stage 2 (`stage2.asm, stage2_load.c`)
+
+- Set up segment registers
+- Load the kernel ELF header
+- Parse the program headers, and load all `PT_LOAD` segments from disk
+- Jump to the kernel entry