mkimage.c: Split into separate files.

util/grub-mkimagexx.c is included in a special way into mkimage.c.
Interoperation between defines makes this very tricky. Instead
just have a clean interface and compile util/grub-mkimage*.c separately
from mkimage.c

util/grub-mkimage32.c

@@ -0,0 +1,22 @@
+#define MKIMAGE_ELF32 1
+
+# define SUFFIX(x)	x ## 32
+# define ELFCLASSXX	ELFCLASS32
+# define Elf_Ehdr	Elf32_Ehdr
+# define Elf_Phdr	Elf32_Phdr
+# define Elf_Nhdr	Elf32_Nhdr
+# define Elf_Addr	Elf32_Addr
+# define Elf_Sym	Elf32_Sym
+# define Elf_Off	Elf32_Off
+# define Elf_Shdr	Elf32_Shdr
+# define Elf_Rela       Elf32_Rela
+# define Elf_Rel        Elf32_Rel
+# define Elf_Word       Elf32_Word
+# define Elf_Half       Elf32_Half
+# define Elf_Section    Elf32_Section
+# define ELF_R_SYM(val)		ELF32_R_SYM(val)
+# define ELF_R_TYPE(val)		ELF32_R_TYPE(val)
+# define ELF_ST_TYPE(val)		ELF32_ST_TYPE(val)
+#define XEN_NOTE_SIZE 132
+
+#include "grub-mkimagexx.c"