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>

 # Configuration file for sstrip tool facility

 if OBSOLETE && ! BACKEND && ARCH_BINFMT_ELF

 comment "sstrip"

 config SSTRIP

     bool

     prompt "sstrip (OBSOLETE)"

     help

       The sstrip utility, to maximise the striping of ELF binaries

       (executables and libraries).

       Buildroot version, forked off the original from ELFkickers.

       This one is somewhat maintained by the buildroot guys.

       Supports big-endian systems.

       NOTE! Please read.

         sstrip is now deprecated in crosstool-NG, as it has a few

         shortcomings. For example: it breaks for PPC targets, it

         does not build on non-ELF hosts (eg. mingw32, MacOS-X...).

 endif