hawkboard: Update config file to work with common spl framework

The common spl framework expects the u-boot payload size through
CONFIG_SYS_MONITOR_LEN. Define the macro with the u-boot's size. With
this change, CONFIG_SYS_NAND_U_BOOT_SIZE is no longer required. Delete
the same.

Signed-off-by: Tom Rini <trini@ti.com>
Signed-off-by: Sughosh Ganu <urwithsughosh@gmail.com>