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>

 # Super-H specific configuration file

 choice

     bool

     prompt "Variant"

 config ARCH_SH_SH3

     bool

     prompt "sh3"

 config ARCH_SH_SH4

     bool

     prompt "sh4"

 config ARCH_SH_SH4A

     bool

     prompt "sh4a"

 endchoice

 config ARCH_SH_VARIANT

     string

     default "sh3"   if ARCH_SH_SH3

     default "sh4"   if ARCH_SH_SH4

     default "sh4a"  if ARCH_SH_SH4A