bootstub: get system memory map by int 15h on Baytrail platform

BZ: 96847

There is no SFI MMAP table any more for Baytrail platform.
Now we can get system memory map information by int 15h which returns
an e820 table with memory map entries.
We have to switch to real mode to call int 15h then switch back to
protected mode.
This patch implements the real mode int 15 call in assembly language
but exported a C style function:
int get_e820_by_bios(void *e820_buf);
See details in e820_bios.S.

Change-Id: I33dbeaaa58da217e74e015f0d911896500043d17
Signed-off-by: Bin Gao <[email protected]>
Reviewed-on: http://android.intel.com:8080/99216
Reviewed-by: Balestriere, VianneyX <[email protected]>
Tested-by: Balestriere, VianneyX <[email protected]>
5 files changed