Changes to move hawkboard to the new spl infrastructure

This patch moves hawkboard to the new spl infrastructure from the
older nand_spl one.

Removed the hawkboard_nand_config build option -- The spl code now
gets compiled with hawkboard_config, after building the main u-boot
image, using the CONFIG_SPL_TEXT_BASE. Modified the README.hawkboard
to reflect the same.

Signed-off-by: Sughosh Ganu <urwithsughosh@gmail.com>
Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Christian Riesch <christian.riesch@omicron.at>
Cc: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Cc: Tom Rini <trini@ti.com>
Acked-by: Christian Riesch <christian.riesch@omicron.at>