Fix the makefile template

 - Corrects a python barf when something in features is None
 - Corrects makefile to prevent barf when using make -f

Author: theotherjimmy

tools/export/gcc_arm_common.tmpl

@@ -13,7 +13,8 @@ endif
 ifeq (,$(filter .build,$(notdir $(CURDIR))))
 .SUFFIXES:
 OBJDIR := .build
-MAKETARGET = $(MAKE) --no-print-directory -C $(OBJDIR) -f $(CURDIR)/Makefile \
+mkfile_path := $(abspath $(lastword $(MAKEFILE_LIST)))
+MAKETARGET = $(MAKE) --no-print-directory -C $(OBJDIR) -f $(mkfile_path) \
 		SRCDIR=$(CURDIR) $(MAKECMDGOALS)
 .PHONY: $(OBJDIR) clean
 all:

tools/toolchains/__init__.py

@@ -147,7 +147,7 @@ def relative_to(self, base, dot=False):
             v = [rel_path(f, base, dot) for f in getattr(self, field)]
             setattr(self, field, v)
 
-        self.features = {k: f.relative_to(base, dot) for k, f in self.features.iteritems()}
+        self.features = {k: f.relative_to(base, dot) for k, f in self.features.iteritems() if f}
 
         if self.linker_script is not None:
             self.linker_script = rel_path(self.linker_script, base, dot)
@@ -160,7 +160,7 @@ def win_to_unix(self):
             v = [f.replace('\\', '/') for f in getattr(self, field)]
             setattr(self, field, v)
 
-        self.features = {k: f.win_to_unix() for k, f in self.features.iteritems()}
+        self.features = {k: f.win_to_unix() for k, f in self.features.iteritems() if f}
 
         if self.linker_script is not None:
             self.linker_script = self.linker_script.replace('\\', '/')