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>

 # Binary utilities menu

 menu "Binary utilities"

 choice

     bool

     prompt "Binary format:"

 if ARCH_USE_MMU || BARE_METAL

 config ARCH_BINFMT_ELF

     bool

     prompt "ELF"

     help

       This will make your system build ELF exectubales,

       suitable for architectures with an MMU.

 endif # ARCH_USE_MMU

 if ! ARCH_USE_MMU

 config ARCH_BINFMT_FLAT

     bool

     prompt "Flat"

     help

       This will build flat binaries, suitable for

       MMU-less architectures.

 config ARCH_BINFMT_FDPIC

     bool

     prompt "FD_PIC ELF"

     help

       This will build FD_PIC ELF binaries, suitable for

       MMU-less architectures that still require to use

       shared libraries (FIXME).

 endif # ! ARCH_USE_MMU

 endchoice

 source "config/binutils/binutils.in"

 source "config/binutils/elf2flt.in"

 source "config/binutils/sstrip.in"

 endmenu