ntp: use SPDX short identifier for ntp license

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

package/ntp/ntp.mk

@@ -8,7 +8,7 @@ NTP_VERSION_MAJOR = 4.2
 NTP_VERSION = $(NTP_VERSION_MAJOR).8p10
 NTP_SITE = https://www.eecis.udel.edu/~ntp/ntp_spool/ntp4/ntp-$(NTP_VERSION_MAJOR)
 NTP_DEPENDENCIES = host-pkgconf libevent openssl $(if $(BR2_PACKAGE_BUSYBOX),busybox)
-NTP_LICENSE = ntp license
+NTP_LICENSE = NTP
 NTP_LICENSE_FILES = COPYRIGHT
 NTP_CONF_ENV = ac_cv_lib_md5_MD5Init=no
 NTP_CONF_OPTS = \