configs: sei510/sei610: don't use hard-coded gpt uuids

doc/README.gpt states:

> The fields 'uuid' and 'uuid_disk' are optional if CONFIG_RANDOM_UUID is
> enabled. A random uuid will be used if omitted or they point to an empty/
> non-existent environment variable. The environment variable will be
> set to the generated UUID.  The 'gpt guid' command reads the current
> value of the uuid_disk from the GPT.

Since we have CONFIG_RANDOM_UUID=y, remove the hard-coded uuids
and use meaningful variable names instead.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>