support blackfin board initialization in generic board_f

- init hardware watchdog if applicable
- use CONFIG_SYS_MONITOR_LEN as the gd monitor len for Blackfin
- reserve u-boot memory at the top field of the RAM for Blackfin
- avoid refer to CONFIG_SYS_MONITOR_LEN, which is not defined by Blackfin

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>