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Add DISCARD directives to the linker scripts and also -marm option
to GCC command line. Without this, certain toolchains may build
Thumb2 code by default. Also junk sections may be present in the
final binary and they need to be removed.

This helps at least when using the following Linaro toolchain:
gcc-linaro-5.1-2015.08-x86_64_arm-linux-gnueabihf.tar.xz
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Reviewed-by: Bernhard Nortmann <bernhard.nortmann@web.de>