image-fit: Fix fit_get_node_from_config semantics

Commit bac17b78 ("image-fit: switch ENOLINK to ENOENT") changed
fit_get_node_from_config to return -ENOENT when a property doesn't
exist, but didn't change any of its callers which check return values.
Notably it didn't change boot_get_ramdisk, which leads to U-Boot failing
to boot FIT images which don't include ramdisks with the following
message:

  Ramdisk image is corrupt or invalid

It also didn't take into account that by returning -ENOENT to denote the
lack of a property we lost the ability to determine from the return
value of fit_get_node_from_config whether it was the property or the
configuration node that was missing, which may potentially lead callers
to accept invalid FIT images.

Fix this by having fit_get_node_from_config return -EINVAL when the
configuration node isn't found and -ENOENT when the property isn't
found, which seems to make semantic sense. Callers that previously
checked for -ENOLINK are adjusted to check for -ENOENT, which fixes the
breakage introduced by commit bac17b78 ("image-fit: switch ENOLINK
to ENOENT").

The only other user of the return fit_get_node_from_config return value,
indirectly, is bootm_find_os which already checked for -ENOENT. From a
read-through of the code I suspect it ought to have been checking for
-ENOLINK prior to bac17b78 ("image-fit: switch ENOLINK to ENOENT")
anyway, which would make it right after this patch, but this would be
good to get verified by someone who knows this x86 code or is able to
test it.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Jonathan Gray <jsg@jsg.id.au>
Cc: Marek Vasut <marex@denx.de>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: George McCollister <george.mccollister@gmail.com>
Tested-by: George McCollister <george.mccollister@gmail.com>