package: use SPDX short identifier for libpng license

We want to use SPDX identifier for license string as much as possible.
SPDX short identifier for libpng license is Libpng.

Signed-off-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libpng/libpng.mk

@@ -8,7 +8,7 @@ LIBPNG_VERSION = 1.6.28
 LIBPNG_SERIES = 16
 LIBPNG_SOURCE = libpng-$(LIBPNG_VERSION).tar.xz
 LIBPNG_SITE = http://downloads.sourceforge.net/project/libpng/libpng${LIBPNG_SERIES}/$(LIBPNG_VERSION)
-LIBPNG_LICENSE = libpng license
+LIBPNG_LICENSE = Libpng
 LIBPNG_LICENSE_FILES = LICENSE
 LIBPNG_INSTALL_STAGING = YES
 LIBPNG_DEPENDENCIES = host-pkgconf zlib

package/python-pysmb/python-pysmb.mk

@@ -7,7 +7,7 @@
 PYTHON_PYSMB_VERSION = 1.1.19
 PYTHON_PYSMB_SOURCE = pysmb-$(PYTHON_PYSMB_VERSION).tar.gz
 PYTHON_PYSMB_SITE = https://pypi.python.org/packages/f9/e7/1fd7faaa946cc6b43ce85bb7a177b75a4718d9c5e291201fec00112b497c
-PYTHON_PYSMB_LICENSE = libpng license
+PYTHON_PYSMB_LICENSE = Libpng
 PYTHON_PYSMB_LICENSE_FILES = LICENSE
 PYTHON_PYSMB_SETUP_TYPE = setuptools