Fix build for individual profiles with dashes in names

Config symbols can have regular dashes, e.g.

CONFIG_TARGET_ramips_rt305x_UR-336UN=y

So no substitution should be performed on the last part of the symbol.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39242 3c298f89-4303-0410-b956-a3cf2f4a3e73