With TARGET_ROOTFS_INITRAMFS_SEPARATE, the initramfs kernel differs
from the regular kernel only in CONFIG_RD_<ALGO>. Both are built in the
same tree, so every build switches the kernel config twice and relinks
vmlinux twice, even if nothing changed.
Give the regular kernel the same ramdisk support and use its image for
the initramfs as is. The initramfs step now only creates the initrd.
Remove the separate initramfs branches of Kernel/SetInitramfs, as it is
no longer called in this mode.
Signed-off-by: John Crispin <john@phrozen.org>