The function image_boot_mode_id() converts the name of a boot media
into the corresponding Marvell specific code. However, 0 that we
currently used to indicate that the boot media name wasn't found,
could potentially be a valid value. So instead we use -1 to indicate a
failure.

This is also done in preparation to the introduction of
image_nand_ecc_mode_id(), which will convert a NAND ECC mode name into
the corresponding identifier. And in this case 0 is a valid identifier
of a NAND ECC mode, so we cannot use it to indicate a failure. Since
we want image_boot_mode_id() and image_nand_ecc_mode_id() to have a
consistent behavior, we change the former in this commit. The latter
is introduced in the next commit.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>