Make ubootenv.d work on Wandboard (Closes: #884375).

To enable graphics on wandboard you need to have contiguous memory
available, eg. pass cma=256M on the kernel command line.
Unfortunately dropping 'setenv bootargs ${bootargs} cma=256M'
into /etc/flash-kernel/ubootenv.d/cma currently doesn't work
because the bootscr.wandboard template doesn't contain the
needed @@UBOOT_ENV_EXTRA@@ marker for where to insert...
This change fixes the template so we can use ubootenv.d dropins
for wandboard.