Separate more clearly bootloader from xv6 by renaming multiboot.S to entry.S etc.

Maybe the string boot shouldn't appear in xv6 code?

entry.S

@@ -0,0 +1,60 @@
+# Multiboot header, for multiboot boot loaders like GNU Grub.
+# http://www.gnu.org/software/grub/manual/multiboot/multiboot.html
+#
+# Using GRUB 2, you can boot xv6 from a file stored in a
+# Linux file system by copying kernel or kernelmemfs to /boot
+# and then adding this menu entry:
+#
+# menuentry "xv6" {
+# 	insmod ext2
+# 	set root='(hd0,msdos1)'
+# 	set kernel='/boot/kernel'
+# 	echo "Loading ${kernel}..."
+# 	multiboot ${kernel} ${kernel}
+# 	boot
+# }
+
+#include "asm.h"
+#include "memlayout.h"
+#include "mmu.h"
+
+#define STACK 4096
+
+# Multiboot header.  Data to direct multiboot loader.
+.p2align 2
+.text
+.globl multiboot_header
+multiboot_header:
+  #define magic 0x1badb002
+  #define flags (1<<16 | 1<<0)
+  .long magic
+  .long flags
+  .long (-magic-flags)
+  .long multiboot_header  # beginning of image
+  .long multiboot_header
+  .long edata
+  .long end
+  .long multiboot_entry
+
+# Multiboot entry point.  Machine is mostly set up.
+.globl multiboot_entry
+multiboot_entry:
+  movl    $(V2P_WO(bootpgdir)), %eax
+  movl    %eax, %cr3
+  # Turn on paging.
+  movl    %cr0, %eax
+  orl     $(CR0_PE|CR0_PG|CR0_WP), %eax
+  movl    %eax, %cr0
+
+  # now switch to using addresses above KERNBASE
+  # call addresses are pc-relative so we jump though this hoop:
+  mov     $relocated, %eax
+  jmp     *%eax
+relocated:
+  # Set up the stack pointer and call into C.
+  movl $(stack + STACK), %esp
+  call main
+spin:
+  jmp spin
+
+.comm stack, STACK