x86: Kconfig: Add location options for 16/32-bit init

At present all 16/32-bit init is controlled by CONFIG_X86_RESET_VECTOR. If
this is enabled, then U-Boot is the 'first' boot loader and handles execution
from the reset vector through to U-Boot's command prompt. If it is not
enabled then U-Boot starts at the 32-bit entry and skips most of its init,
assuming that the previous boot loader has done this already.

With the move to suport 64-bit operation, we have more cases to consider.
The 16-bit and 32-bit init may be in SPL rather than in U-Boot proper.

Add Kconfig options which control the location of the 16-bit and the 32-bit
init. These are not intended to be user-setting except for experimentation.
Their values should be determined by whether 64-bit U-Boot is used.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>