oseama: support creating single Seama entity

This will be needed to handle sysupgrade on NAND in a smart way. We'll
need to extract kernel our of provided Seama container, put in in a new
Seama entity and flash.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48552 3c298f89-4303-0410-b956-a3cf2f4a3e73