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>

 # MIPS specific config options

 choice

     bool

     prompt "ABI"

 config ARCH_mips_o32

     bool

     prompt "o32"

     depends on ARCH_32

     help

       This is the -mabi=32 gcc option.

 config ARCH_mips_n32

     bool

     prompt "n32"

     depends on ARCH_64

     help

       This is the -mabi=n32 gcc option.

 config ARCH_mips_n64

     bool

     prompt "n64"

     depends on ARCH_64

     help

       This is the -mabi=64 gcc option.

 # Not supported on Linux:

 # o64   : seems related to *BSD

 # eabi  : seems related to bare-metal

 endchoice

 config ARCH_mips_ABI

     string

     default "32"    if ARCH_mips_o32

     default "n32"   if ARCH_mips_n32

     default "64"    if ARCH_mips_n64