PX4: conditionally build auth command and libraries

this makes it possible to build with upstream master

mk/PX4/config_px4fmu-v2_APM.mk

@@ -51,7 +51,11 @@ MODULES		+= systemcmds/reboot
 MODULES		+= systemcmds/top
 MODULES		+= systemcmds/tests
 MODULES		+= systemcmds/nshterm
+# the conditional allows for building with upstream master
+# which doesn't have auth
+ifneq ($(wildcard systemcmds/auth),)  
 MODULES		+= systemcmds/auth
+endif
 MODULES         += systemcmds/mtd
 
 #
@@ -61,8 +65,12 @@ MODULES		+= modules/systemlib
 MODULES		+= modules/systemlib/mixer
 MODULES		+= modules/uORB
 MODULES		+= lib/mathlib/math/filter
+# the conditional allows for building with upstream master
+# which doesn't have libtomcrypt and libtomfastmath
+ifneq ($(wildcard modules/libtomfastmath),)  
 MODULES	        += modules/libtomfastmath
 MODULES         += modules/libtomcrypt
+endif
 MODULES		+= lib/conversion
 
 #