frr: makefile cleanup and misc fixes

fix mips runtime by backporting some yang changes from master
added commited fixes to 7.3
also add option for snmp support

Signed-off-by: Lucian Cristian <lucian.cristian@gmail.com>

net/frr/patches/098-fix_mips_libyang.patch

@@ -0,0 +1,14 @@
+--- a/lib/northbound.h
++++ b/lib/northbound.h
+@@ -504,11 +504,7 @@ struct frr_yang_module_info {
+ 
+ 		/* Priority - lower priorities are processed first. */
+ 		uint32_t priority;
+-#if defined(__GNUC__) && ((__GNUC__ - 0) < 5) && !defined(__clang__)
+ 	} nodes[YANG_MODULE_MAX_NODES + 1];
+-#else
+-	} nodes[];
+-#endif
+ };
+ 
+ /* Northbound error codes. */