init-install: Strip partition number from live_dev_name

init-install: Strip partition number from live_dev_name

This is needed in case the boot disk was created with mkdiskimage.
In that case the parameter passed is a variant of /dev/sda4 which
includes the partition number.  Without this change this install script
will offer to install onto the live media.

(From OE-Core rev: 9f6d7d42eaad225698de730d5c76bfe9523f4a78)

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>