meta/meta-selftest/meta-skeleton: Update LICENSE variable to use SPDX license identifiers

An automated conversion using scripts/contrib/convert-spdx-licenses.py to
convert to use the standard SPDX license identifiers. Two recipes in meta-selftest
were not converted as they're that way specifically for testing. A change in
linux-firmware was also skipped and may need a more manual tweak.

(From OE-Core rev: ceda3238cdbf1beb216ae9ddb242470d5dfc25e0)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-skeleton/recipes-kernel/hello-mod/hello-mod_0.1.bb

@@ -1,6 +1,6 @@
 SUMMARY = "Example of how to build an external Linux kernel module"
 DESCRIPTION = "${SUMMARY}"
-LICENSE = "GPLv2"
+LICENSE = "GPL-2.0-only"
 LIC_FILES_CHKSUM = "file://COPYING;md5=12f884d2ae1ff87c09e5b7ccc2c4ca7e"
 
 inherit module

meta-skeleton/recipes-skeleton/hello-autotools/hello_2.10.bb

@@ -1,6 +1,6 @@
 DESCRIPTION = "GNU Helloworld application"
 SECTION = "examples"
-LICENSE = "GPLv3"
+LICENSE = "GPL-3.0-only"
 LIC_FILES_CHKSUM = "file://COPYING;md5=d32239bcb673463ab874e80d47fae504"
 
 SRC_URI = "${GNU_MIRROR}/hello/hello-${PV}.tar.gz"

meta-skeleton/recipes-skeleton/service/service_0.1.bb

@@ -1,7 +1,7 @@
 SUMMARY = "The canonical example of init scripts"
 SECTION = "base"
 DESCRIPTION = "This recipe is a canonical example of init scripts"
-LICENSE = "GPLv2"
+LICENSE = "GPL-2.0-only"
 LIC_FILES_CHKSUM = "file://${WORKDIR}/COPYRIGHT;md5=349c872e0066155e1818b786938876a4"
 
 SRC_URI = "file://skeleton \