Converted/fixed compilation with gcc-arm-embedded

When compiling with the newest version of libopencm and gcc arm embedded, we were occupying more then the available 8kb of space reserved for the bootloader.

Changed the linker parameters to use the nano.specs setup so that the binary fits in the first sector of memory.

The use of nano.specs will break compilation on older (linaro for example) compilers that don't support that function.

As paparazzi is moving to use gcc-arm-embedded as it's default compiler this should not be an issue...

Please test it so we can merge it soon. :)