We will need CONFIG_SYS_MALLOC_F_LEN to use serial uclass.
So we shall undefine CONFIG_SYS_GENERIC_GLOBAL_DATA, and
call board_init_f_mem() to allocates early malloc() memory
with size of CONFIG_SYS_MALLOC_F_LEN in board_f.c.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Marek Vasut <marex@denx.de>