base-files: default_do_upgrade: accept & forward image extraction command

get_image allows passing 2nd argument that is used in a pipe for
extracting firmware from a non-native format. By accepting such command
in default_do_upgrade we allow platforms to use this helper for vendor
specific images.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45900 3c298f89-4303-0410-b956-a3cf2f4a3e73