SPL: do not use fix value for u-boot size

If an u-boot image is not found, SPL thinks to load a bare
u-boot.bin image with a maximum size of 200KB.
Use CONFIG_SYS_MONITOR_LEN instead.

Signed-off-by: Stefan Roese <stefan.roese@gmail.com>
Signed-off-by: Stefano Babic <sbabic@denx.de>
Signed-off-by: Tom Rini <trini@ti.com>