x86: call FspInitWrapper in start.S

FspInit() is the 2nd stage initialization called by the bootloader
after TempRamInit(). The FspInit() requires a new stack address
provided by the bootloader after the system memory is initialized.
Since U-Boot will do its relocation in the later phase, here just
use a predefined temporary address from the configuration file.

Note we must re-initialize EBX with the correct boot flag before
jumping to U-Boot board_init_f() as the content of EBX has already
been corrupted through the FspInit() call.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>