sh: boot: add intermediate vmlinux.bin* to targets instead of extra-y

You do not need to build all of vmlinux.bin*

They are built on demand as prerequsites of uImage.bin*, hence should
be added to targets instead of extra-y.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Rich Felker <dalias@libc.org>