efi_selftest: test start image

This pair of tests checks the StartImage boot service.

Each test loads an EFI application into memory and starts it.
One returns by calling the Exit boot service. The other returns directly.

The tests are not built on x86_64 because the relocation code for the efi
binary cannot be created.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>