gdb: add support for Python in target gdb

This commit adds a new option BR2_PACKAGE_GDB_PYTHON to enable Python
support in the target gdb. Since we can assume that the user will be
aware that Python is needed to get Python support in gdb, we chose to
use a "depends on" dependency instead of a "select" dependency.

The other weird thing is the need for a wrapper shell script to
replace gdb's provided python-config.py script. See the shell script
comment itself for all the details.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>