meta-ethanolx: Add support for Legacy/EFI selector

AMD EthanolX CRB is a board with x86-64 CPU, which means it can support
both EFI and Legacy boot.
Add 'boot_type' PACKAGECONFIG option for the phosphor-settings-manager
package to support EFI/Legacy selector in the BMC boot override feature.
This setting is designed for the possibility of setting boot override
as:
- "PC compatible" boot (legacy)
- Extensible Firmware Interface Boot (EFI)

Change-Id: I173c5b439b929e80f3a106f5e7775862f889ee16
Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>