debug/cross-gdb: check host dependencies

Cross-gdb depends on expat and python. If either is missing, cross-gdb will
build successfully, but lacking some features.

Especially, if expat is missing, cross-gdb will be unable to parse the target
description, which may lead to runtime malfunctions and the following GDB
warning:
"Can not parse XML target description; XML support was disabled at compile time"

Hence, expat should be considered mandatory.

On the other hand, the features missing without python are not critical, so
python should not be considered mandatory.

This patch does the following:
- At configure time, warn the user if either expat or python is missing.
- In menuconfig, disable the static build options regarding cross-gdb if no
static version of expat is available, and disable cross-gdb if expat is
missing.

Signed-off-by: "Benoît THÉBAUDEAU" <benoit.thebaudeau@advansee.com>
[yann.morin.1998@anciens.enib.fr: add comment for impossible static cross-gdb]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Menu for the native gdbserver

 config GDB_GDBSERVER

     bool

     prompt "gdbserver"

     depends on ! BARE_METAL

     help

       Build and install a gdbserver for the target, to run on the target.

 if GDB_GDBSERVER

 config GDB_GDBSERVER_STATIC

     bool

     prompt "Build a static gdbserver"

     default y

     help

       In case you have trouble with dynamic loading of shared libraries,

       you will find that a static gdbserver comes in handy.

 endif # GDB_GDBSERVER

 if BARE_METAL

 comment "In bare-metal, you'll need to   "

 comment "provide your own gdbserver stub."

 endif # BARE_METAL