mender: fix check-package warnings

Fixes:

package/mender/Config.in:7: help text: <tab><2 spaces><62 chars> (http://nightly.buildroot.org/#writing-rules-config-in)
package/mender/Config.in:8: help text: <tab><2 spaces><62 chars> (http://nightly.buildroot.org/#writing-rules-config-in)
package/mender/mender.mk:8: remove default value of _SOURCE variable (http://nightly.buildroot.org/#generic-package-reference)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/mender/Config.in

@@ -4,10 +4,10 @@ config BR2_PACKAGE_MENDER
 	depends on BR2_PACKAGE_HOST_GO_CGO_LINKING_SUPPORTS
 	depends on BR2_TOOLCHAIN_HAS_THREADS
 	help
-	  Mender is an open source over-the-air (OTA) software updater for
-	  embedded Linux devices. Mender comprises a client running at the
-	  embedded device, as well as a server that manages deployments
-	  across many devices.
+	  Mender is an open source over-the-air (OTA) software updater
+	  for embedded Linux devices. Mender comprises a client
+	  running at the embedded device, as well as a server that
+	  manages deployments across many devices.
 
 	  https://github.com/mendersoftware/mender
 

package/mender/mender.mk

@@ -5,7 +5,6 @@
 ################################################################################
 
 MENDER_VERSION = 1.4.0
-MENDER_SOURCE = mender-$(MENDER_VERSION).tar.gz
 MENDER_SITE = $(call github,mendersoftware,mender,$(MENDER_VERSION))
 
 define MENDER_INSTALL_CONFIG_FILES