kernel.mk: allow to build linux rules using kernel_xxx.
kernel.mk: allow to build linux rules using kernel_xxx.

Make kernel_xxx an alias to linux rules, so that:
$ make kernel_zImage
does the same thing that:
$ make linux_zImage

Signed-off-by: Gaël PORTAY <1666f60cfa201a60b0ea694168a0b6932673170a@gmail.com>

gazoo74/initramfs,gazoo74/initramfs