boot: separate BIOS from UEFI

boot/bios/ now holds the MBR, the El Torito stub and the real-mode stage
2; boot/uefi/ is empty and documented.

Separate directories rather than #ifdefs, because the two share almost
nothing. A BIOS loader lives in 16-bit real mode, reaches the firmware
through software interrupts, and walks itself up into long mode. A UEFI
loader is a 64-bit PE that the firmware has already placed in long mode
with paging on, calling services through a table of function pointers.
What they do share is the hand-off contract, which boot/README.md now
writes down: ELF segments placed, long mode, a 24-byte-entry memory map
at E820_MAP_ADDR, framebuffer geometry at VBE_INFO_ADDR.

That contract is the point of documenting it now. UEFI's GetMemoryMap
returns EFI_MEMORY_DESCRIPTORs, and the right move is to translate them
into the entries kernel/arch/x86_64/e820.c already parses rather than
teach the kernel a second format.

Build output paths under $(BUILD)/boot are unchanged; only sources moved.

kernel/arch/x86_64/e820.c

@@ -7,7 +7,7 @@
 // every byte above 224 MB is invisible. Measured before this file existed:
 // SchOS booted at 224 MB and triple faulted at 192 MB, for no reason at all.
 //
-// The firmware has known the answer the whole time. boot/stage2.S asks it with
+// The firmware has known the answer the whole time. boot/bios/stage2.S asks it with
 // INT 0x15 / E820 in real mode -- the same interface Linux's boot stub and GRUB
 // use -- and leaves the result in low memory. stage2main.c already parses it to
 // print detected RAM on the splash. The kernel simply never read it.
@@ -23,7 +23,7 @@
 uint64 phystop = PHYSTOP_FALLBACK;
 
 // Where stage2 left the map. Must match E820_COUNT_ADDR / E820_MAP_ADDR in
-// boot/stage2main.c, which are themselves relative to VBE_INFO_ADDR.
+// boot/bios/stage2main.c, which are themselves relative to VBE_INFO_ADDR.
 #define E820_COUNT_ADDR (0x30000 + 0x200)
 #define E820_MAP_ADDR   (0x30000 + 0x208)
 

kernel/core/main.c

@@ -12,12 +12,12 @@ static void startothers(void);
 static void mpmain(void)  __attribute__((noreturn));
 extern char end[]; // first address after kernel loaded from ELF file
 
-// Set by stage2main.c (boot/stage2main.c, BOOT_VERBOSE_FLAG_ADDR) when the
+// Set by stage2main.c (boot/bios/stage2main.c, BOOT_VERBOSE_FLAG_ADDR) when the
 // user picks "Boot SchOS (verbose)" from the F8 boot menu; read once here,
 // before anything else, so subsystem init can log its own steps.
 int boot_verbose;
 
-// Must match BOOT_VERBOSE_FLAG_ADDR in boot/stage2main.c exactly: just past
+// Must match BOOT_VERBOSE_FLAG_ADDR in boot/bios/stage2main.c exactly: just past
 // the VBE info block, not 0x7000 (startothers() below uses that address for
 // the AP boot trampoline).
 #define BOOT_VERBOSE_FLAG_ADDR (0x30000 + 0x100)

kernel/dev/vga.c

@@ -39,7 +39,7 @@ vga_set_bg(uint32 color)
   vga_bg = color;
 }
 
-// Written by boot/stage2.S's VBE call and read by boot/stage2main.c under
+// Written by boot/bios/stage2.S's VBE call and read by boot/bios/stage2main.c under
 // the same name (VBE_INFO_ADDR) — not 0x9000, since stage2's own code now
 // runs up to nearly 0x9000 once the boot-menu/progress-bar logic is
 // included, which would otherwise clobber a mode info block placed there.
@@ -391,7 +391,7 @@ vga_text_at(int x, int y, const char *s, uint32 color)
 // --- Logo ------------------------------------------------------------------
 //
 // The boot splash's logo is drawn by the *loader*, not here -- see splash() in
-// boot/stage2main.c. It has to be, because the kernel does not reach vga_init()
+// boot/bios/stage2main.c. It has to be, because the kernel does not reach vga_init()
 // until roughly the last 300ms of a 2.8s boot: drawing it here put the logo on
 // screen for one frame before console_takeover() wiped it, which is worse than
 // not having one.