boot, package: use SPDX short identifier for GPLv3/GPLv3+

We want to use SPDX identifier for license string as much as possible.
SPDX short identifier for GPLv3/GPLv3+ is GPL-3.0/GPL-3.0+.

This change is done using following command.
find . -name "*.mk" | xargs sed -ri '/LICENSE( )?[\+:]?=/s/\<GPLv3\>/GPL-3.0/g'

Signed-off-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
diff --git a/package/triggerhappy/triggerhappy.mk b/package/triggerhappy/triggerhappy.mk
index 5a29ca7..a57f1a7 100644
--- a/package/triggerhappy/triggerhappy.mk
+++ b/package/triggerhappy/triggerhappy.mk
@@ -6,7 +6,7 @@
 
 TRIGGERHAPPY_VERSION = 7e5abc69f215678e93a6b999524981c8b40bdcd9
 TRIGGERHAPPY_SITE = $(call github,wertarbyte,triggerhappy,$(TRIGGERHAPPY_VERSION))
-TRIGGERHAPPY_LICENSE = GPLv3+
+TRIGGERHAPPY_LICENSE = GPL-3.0+
 TRIGGERHAPPY_LICENSE_FILES = COPYING
 
 define TRIGGERHAPPY_BUILD_CMDS