package/systemd: don't treat ld warnings as errors

Nios2 ld emits warnings like:
'
/home/buildroot/autobuild/instance-2/output-1/host/opt/ext-toolchain/bin/../lib/gcc/nios2-linux-gnu/7.3.1/../../../../nios2-linux-gnu/bin/ld: FDE encoding in /home/buildroot/autobuild/instance-2/output-1/host/opt/ext-toolchain/bin/../lib/gcc/nios2-linux-gnu/7.3.1/libgcc.a(_umoddi3.o)(.eh_frame) prevents .eh_frame_hdr table being created.
'
Since '-Wl,--fatal-warnings' is passed by default, build fails, so don't
treat warnings as errors by appending "-Wl,--no-fatal-warnings" to
SYSTEMD_LDFLAGS the same way I've done for Microblaze in commit:
https://git.buildroot.net/buildroot/commit/?id=cc53d5357d84ff9b78d76e45ce216061df8a37c1

Fixes:
http://autobuild.buildroot.net/results/5fc884e11e46ab165ea21c8e8f2901516d79ff80/

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/systemd/systemd.mk

@@ -90,6 +90,11 @@ SYSTEMD_CONF_OPTS += \
 	-Dumount-path=/usr/bin/umount \
 	-Dutmp=false
 
+ifeq ($(BR2_nios2),y)
+# Nios2 ld emits warnings, make warnings not to be treated as errors
+SYSTEMD_LDFLAGS = $(TARGET_LDFLAGS) -Wl,--no-fatal-warnings
+endif
+
 ifeq ($(BR2_PACKAGE_ACL),y)
 SYSTEMD_DEPENDENCIES += acl
 SYSTEMD_CONF_OPTS += -Dacl=true