linux: make vmlinux target available to all architectures

The 'vmlinux' target in the linux build directory is a generic target. It may
not be directly bootable for all architectures, but for projects where a custom
bootloader is used, it can be of value.

Previously, this target was only available for mips architectures. This patch
removes this restriction.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>