lantiq: use sysupgrade.bin postfix for tplink images

Use the same name for TP-Link images as it was with the old image build
code.

Move the BOARD_ID export to the TP-Link image build recipe, to indicate
that the variable is only related in this context.

Signed-off-by: Mathias Kresin <openwrt@kresin.me>

SVN-Revision: 49279