build: Enable GNU null-dereference warning

Add -Wnull-dereference to GNU toolchain for additional null pointer
warnings.

Signed-off-by: Gabor Toth <gabor.toth@arm.com>
Change-Id: I39d81d561255d23e32cccc5e984f9b337befc95a
diff --git a/toolchain_GNUARM.cmake b/toolchain_GNUARM.cmake
index d8c0e35..e1a1c1d 100644
--- a/toolchain_GNUARM.cmake
+++ b/toolchain_GNUARM.cmake
@@ -114,6 +114,7 @@
     -Wall
     -Wno-format
     -Wno-unused-but-set-variable
+    -Wnull-dereference
     -c
     -fdata-sections
     -ffunction-sections