zynq: Extend kernel image size to 20MB

Extend max kernel image size. Gunzip is checking
this value. If kernel is larger, message below is shown.

Uncompressing Kernel Image ... Error: inflate() returned -5
GUNZIP: uncompress, out-of-mem or overwrite error -
must RESET board to recover

Signed-off-by: Michal Simek <michal.simek@xilinx.com>