efi_selftest: allow to select a single test for execution

Environment variable efi_selftest is passed as load options
to the selftest application. It is used to select a single
test to be executed.

The load options are an UTF8 string. Yet I decided to keep
the name propertiy of the tests as char[] to reduce code
size.

Special value 'list' displays a list of all available tests.

Tests get an on_request property. If this property is set
the tests are only executed if explicitly requested.

The invocation of efi_selftest is changed to reflect that
bootefi selftest with efi_selftest = 'list' will call the
Exit bootservice.

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