images imagebuilder: Allow to add sanitized extra name

images imagebuilder: Allow to add sanitized extra name

For final output image names allow user to add an
extra string (which is sanitized).  This is particularly
useful with ImageBuilder where you may generate multiple
images from the same base and for the same board,
with different package selections and additional files
(via FILES=).

Signed-off-by: Daniel Dickinson <openwrt@daniel.thecshore.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48083 3c298f89-4303-0410-b956-a3cf2f4a3e73