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>

doc/README.gpt

@@ -267,6 +267,8 @@ Some strings can be also used at the place of known GUID :
 	           (0657FD6D-A4AB-43C4-84E5-0933C84B4F4F)
 	"lvm"    = PARTITION_LINUX_LVM_GUID
 	           (E6D6D379-F507-44C2-A23C-238F2A3DF928)
+	"u-boot-env"      = PARTITION_U_BOOT_ENVIRONMENT
+	                    (3DE21764-95BD-54BD-A5C3-4ABE786F38A8)
 
     "uuid_disk=...;name=u-boot,size=60MiB,uuid=...;
 	name=kernel,size=60MiB,uuid=...,type=linux;"

include/part_efi.h

@@ -56,6 +56,9 @@
 #define PARTITION_LINUX_LVM_GUID \
 	EFI_GUID( 0xe6d6d379, 0xf507, 0x44c2, \
 		0xa2, 0x3c, 0x23, 0x8f, 0x2a, 0x3d, 0xf9, 0x28)
+#define PARTITION_U_BOOT_ENVIRONMENT \
+	EFI_GUID( 0x3de21764, 0x95bd, 0x54bd, \
+		0xa5, 0xc3, 0x4a, 0xbe, 0x78, 0x6f, 0x38, 0xa8)
 
 /* linux/include/efi.h */
 typedef u16 efi_char16_t;

lib/uuid.c

@@ -96,7 +96,8 @@ static const struct {
 	{"linux",	PARTITION_LINUX_FILE_SYSTEM_DATA_GUID},
 	{"raid",	PARTITION_LINUX_RAID_GUID},
 	{"swap",	PARTITION_LINUX_SWAP_GUID},
-	{"lvm",		PARTITION_LINUX_LVM_GUID}
+	{"lvm",		PARTITION_LINUX_LVM_GUID},
+	{"u-boot-env",	PARTITION_U_BOOT_ENVIRONMENT},
 };
 
 /*