efi_selftest: do not write to linker generated array

Linker generated arrays may be stored in code sections of memory that are
not writable. So let's allocate setup_ok as an array at runtime.

This avoids an illegal memory access observed in the sandbox.

Reported-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>