This adds a tool for installing kernels according to the bootloader
spec. systemd already has a similar tool, but it is limited to installing
kernels on the currently running system. The barebox kernel-install
tool instead can also be used to install kernels on removable media on a
development host for cross development. It is compiled in two variants,
as 'kernel-install' for the host and as 'kernel-install-target' using
$CROSS_COMPILE.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>