Barebox cannot boot the recent mainline Linux kernels for the
i.MX21ADS board anymore when using TFTP, because the heap is too
small.
This is solved by increasing the heapsize to 5Mbyte.
Signed-off-by: Jaccon Bastiaansen <jaccon.bastiaansen@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>