xes: Increase CONFIG_SYS_BOOTM_LEN to 16MB

Increasing CONFIG_SYS_BOOTM_LEN from 8 MB to 16 MB is necessary to
support uncompressing images larger than 8 MB when using the bootm
command.

Note that recent Linux kernels for the 85xx and 86xx map greater than
16MB of memory on bootup, but we use 16MB to maintain compatibility with
older Linux kernels for now.

Signed-off-by: Nate Case <ncase@xes-inc.com>
Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>