toolchain-external: extract installation of gdbserver to separate define

The installation of the gdbserver binary has no relation to the installation
of the target libraries. Moving it to a separate define improves the
understandability of the code and makes later refactoring easier.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
[Thomas:
 - move the BR2_TOOLCHAIN_EXTERNAL_GDB_SERVER_COPY condition as a make
   condition rather than a shell condition, as suggested by Romain
   Naour.
 - rename the TOOLCHAIN_EXTERNAL_INSTALL_GDBSERVER variable to
   TOOLCHAIN_EXTERNAL_INSTALL_TARGET_GDBSERVER as suggested by Arnout.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>