Give kernel image a header.

Kernel image now has a header with version, magic number, and a
pointer to its actual entrypoint. Entry point is now _start in
boot.s, and we now generate versions.s in the build tree for the
version macros.

src/modules/main/boot.s

@@ -0,0 +1,26 @@
+MAGIC	equ  0x600db007			; Popcorn OS header magic number
+
+section .header
+align 4
+global _header
+_header:
+	dd MAGIC
+	db VERSION_MAJOR
+	db VERSION_MINOR
+	dw VERSION_PATCH
+	dd VERSION_GITSHA
+	dq _start
+
+section .text
+align 16
+global _start:function (_start.end - _start)
+_start:
+	extern kernel_main
+	call kernel_main
+
+	cli
+
+.hang:
+	hlt
+	jmp .hang
+.end: