AA: ar71xx: unbreak building images for OpenMesh OM2P/OM2P-LC

AA: ar71xx: unbreak building images for OpenMesh OM2P/OM2P-LC

Backport of r33767.

Commit r33248 introduced a regression by passing the board name plus
'board=' in the third argument. The board name string has to be put
into the image as identifier of the image type.

[juhosg: fix OpenMesh template instead of use of string substitution]

Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@33770 3c298f89-4303-0410-b956-a3cf2f4a3e73