build: only use a build in $SKETCHBOOK for px4

for others it is more convenient to use /tmp, as quite a few scripts
rely on this

mk/environ.mk

@@ -64,7 +64,15 @@ endif
 # Work out where we are going to be building things
 #
 TMPDIR			?=	/tmp
+
+ifneq ($(findstring px4, $(MAKECMDGOALS)),)
+# when building px4 we need all sources to be inside the sketchbook directory
+# as the NuttX build system relies on it
 BUILDROOT		:=	$(SKETCHBOOK)/Build.$(SKETCH)
+else
+BUILDROOT		:=	$(abspath $(TMPDIR)/$(SKETCH).build)
+endif
+
 ifneq ($(findstring CYGWIN, $(SYSTYPE)),)
   # Workaround a $(abspath ) bug on cygwin
   ifeq ($(BUILDROOT),)