debug: provide an option to copy the gdbserver to the target

When an external toolchain is used, it is very likely that it contains
a pre-built version of a gdbserver that has the same version as the
cross-gdb included in the external toolchain. So, we now provide an
option that allows to copy this pre-built gdbserver to the target.

As the location of the gdbserver in the external toolchain is not
standardized, we only support the CodeSourcery and Crosstool-NG
layouts for the moment. Other locations can be added later.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>