Makefile: enable LTO

This rearranges Makefile a bit and enables Link Time Optimisation
which leads to a smaller code (77724 text with gcc 4.7.3).

Unconditionally adding -ggdb3 doesn't affect code size anyhow as all
the debugging info is stored in separate sections.

Build-tested only.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>

Conflicts:

	Makefile
OLIMEXINO - Use I2C2.