Disable Multiboot2 in i386-ieee1275. It didn't actually work, and on
this platform we should support Multiboot1 first.
* conf/i386-ieee1275.rmk (pkglib_MODULES): Remove `multiboot.mod'.
(multiboot_mod_SOURCES, multiboot_mod_CFLAGS)
(multiboot_mod_LDFLAGS, multiboot_mod_ASFLAGS): Remove.