kernel-install: fix return value check for getline ...

getline returns the number of characters read, so check for
ret < 0 instead of ret.

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

kernel-install: Add missing error messages ...

In some cases kernel-install can fail without printing anything. Add
error messages for these cases.

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

add kernel-install tool for bootloader Spec ...

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>