BL: Query and store e820 memory map

- bl stage 1: move gdt+pm setup to stage 2 (avoids mode switching)
- bl stage 2: at entry query e820 table from bios, then setup gdt+pm
- kernel/memmap: pad map entry to 24 bytes, as exported by bl
- kernel/memmap: map copy (gbowne1)
- Makefile: facilitate placing the memory map at a known location
- Update bl docs

bootloader/stage1.asm

@@ -40,11 +40,8 @@ _start:
     call enable_a20
     jc a20_error                ; Jump if A20 enable fails
 
-    ; Setup Global Descriptor Table
-    call setup_gdt
-
-    ; Switch to protected mode and jump to second stage at 0x08:0x7E00
-    call switch_to_pm
+    ; Jump to s2
+    jmp 0x7e00
 
 disk_error:
     mov si, disk_error_msg
@@ -241,30 +238,6 @@ check_a20:
     clc                      ; Clear carry flag to indicate success
     ret
 
-; ----------------------------------------------------------------
-gdt_start:
-    dq 0x0000000000000000         ; Null descriptor
-    dq 0x00CF9A000000FFFF         ; 32-bit code segment (selector 0x08)
-    dq 0x00CF92000000FFFF         ; 32-bit data segment (selector 0x10)
-    dq 0x00009A000000FFFF         ; 16-bit code segment for real mode (selector 0x18)
-
-gdt_descriptor:
-    dw gdt_end - gdt_start - 1
-    dd gdt_start
-gdt_end:
-
-setup_gdt:
-    lgdt [gdt_descriptor]
-    ret
-
-; ----------------------------------------------------------------
-switch_to_pm:
-    cli
-    mov eax, cr0
-    or eax, 1
-    mov cr0, eax
-    jmp 0x08:0x7E00             ; jump to S2
-
 ; ----------------------------------------------------------------
 print_string_16:
 .loop: