firmware/Makefile: don't use distribution's CFLAGS

Set CFLAGS on top of the Makefile, to avoid using the distribution's
default CFLAGS. In debian testing and unstable, -fcf-protection was
recently added to the default CFLAGS, but it cannot be used with the
cross compiler:

  [COMPILING libosmocore/source/backtrace.c]
  cc1: error: '-fcf-protection=full' is not supported for this target
  make[2]: *** [Makefile:270: obj/simtrace/flash_backtrace.o] Error 1

Change-Id: I5f7cd0402f9bb85dd600204fdf8855773ddbeb70
(cherry picked from commit e6e77399d6)

firmware/Makefile

@@ -29,6 +29,13 @@
 #   Makefile for compiling the Getting Started with SAM3S Microcontrollers project
 
 GIT_VERSION=$(shell $(TOP)/git-version-gen $(TOP)/.tarball-version)
+
+CFLAGS = \
+	 -Werror=format-security \
+	 -Wformat \
+	 -g \
+	 $(NULL)
+
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