bootm: Optionally add a root= option to Kernel command line
It becomes a common case that the Kernel is loaded from the filesystem
which later becomes the rootfs. This adds a possibility to let bootm
automatically append the root= option to the kernel command line. This
is done when global.bootm.appendroot is true.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>