boot, package: use SPDX short identifier for LGPLv2.1/LGPLv2.1+

We want to use SPDX identifier for license string as much as possible.
SPDX short identifier for LGPLv2.1/LGPLv2.1+ is LGPL-2.1/LGPL-2.1+.

This change is done using following command.
find . -name "*.mk" | xargs sed -ri '/LICENSE( )?[\+:]?=/s/LGPLv2.1(\+)?/LGPL-2.1\1/g'

Signed-off-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
diff --git a/package/libnl/libnl.mk b/package/libnl/libnl.mk
index f31a92c..6de6825 100644
--- a/package/libnl/libnl.mk
+++ b/package/libnl/libnl.mk
@@ -6,7 +6,7 @@
 
 LIBNL_VERSION = 3.2.29
 LIBNL_SITE = https://github.com/thom311/libnl/releases/download/libnl$(subst .,_,$(LIBNL_VERSION))
-LIBNL_LICENSE = LGPLv2.1+
+LIBNL_LICENSE = LGPL-2.1+
 LIBNL_LICENSE_FILES = COPYING
 LIBNL_INSTALL_STAGING = YES
 LIBNL_DEPENDENCIES = host-bison host-flex host-pkgconf