grub2: disable libdevmapper - fix build when it's available

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40349 3c298f89-4303-0410-b956-a3cf2f4a3e73
grub2: disable mkfont - fix build on Archlinux

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40350 3c298f89-4303-0410-b956-a3cf2f4a3e73