x86: Tidy up selection of building the EFI stub

x86: Tidy up selection of building the EFI stub

At present we use a CONFIG option in efi.h to determine whether we are
building the EFI stub or not. This means that the same header cannot be
used for EFI_LOADER support. The CONFIG option will be enabled for the
whole build, even when not building the stub.

Use a different define instead, set up just for the files that make up the
stub.

Signed-off-by: Simon Glass <[email protected]>
Reviewed-by: Bin Meng <[email protected]>
Signed-off-by: Alexander Graf <[email protected]>