Use an 'initramfs' file if it exists instead of nfsroot. This offers the
possibility to quickly boot complete filesystems as initramfs without
the need to flash an SD-card or similar. If the initramfs does not
exist, it uses the default nfsroot mechanism.

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

The boot-menu-add-entry script no longer exists. Remove it from
boot scripts.

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

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