board: ge: bx50v3, mx53ppd: fix firstboot detection

Use `test' command to test for file existence instead of relying on the
old functionality of the `ext2load' command (which now reports an error
when attempting to load a zero length file).

Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
Signed-off-by: Ian Ray <ian.ray@ge.com>