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>

 # Kernel options

 menu "Operating System"

 # Config option used throughout the config and code to determine wether

 # we have a kernel or not (there might be different bare metal stuff)...

 config BARE_METAL

     bool

 config MINGW32

     bool

 # Each target OS (aka kernel) that support shared libraries can select

 # this, so the user can decide whether or not to build a shared library

 # enabled toolchain

 config KERNEL_SUPPORTS_SHARED_LIBS

     bool

 config KERNEL

     string

 config KERNEL_VERSION

     string

 source "config.gen/kernel.in"

 comment "Common kernel options"

 config SHARED_LIBS

     bool

     prompt "Build shared libraries"

     depends on KERNEL_SUPPORTS_SHARED_LIBS

     default y

     help

       Say 'y' here, unless you don't want shared libraries.

       You might not want shared libraries if you're building for a target that

       don't support it (maybe some nommu targets, for example, or bare metal).

 source "config.gen/kernel.in.2"

 endmenu