doc/README.gpt: define partition type GUID for U-Boot environment

doc/README.gpt: define partition type GUID for U-Boot environment

When setting aside a GPT partition for holding the U-Boot environment,
having a partition type GUID [1] indicating "Linux filesystem" (as
most tools default to) is somewhat misleading - and there's no other
well-known type GUID that is better suited. So to have a canonical
value to put into the type field, define

    3de21764-95bd-54bd-a5c3-4abe786f38a8

to mean a partition holding a U-Boot environment.

This is a v5 namespace-name GUID [2], generated [3] from a namespace
of "25cbcde0-8642-47c6-a298-1a3a57cd256b" and name "U-Boot
environment".

Should future type GUIDs be defined in the context of U-Boot, it's
sensible to use that same namespace GUID.

[1] https://en.wikipedia.org/wiki/GUID_Partition_Table#Partition_type_GUIDs
[2] https://en.wikipedia.org/wiki/Universally_unique_identifier#Versions_3_and_5_(namespace_name-based)
[3] https://www.uuidtools.com/v5

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>