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>

 # Linux kernel options

 if KERNEL_LINUX_INSTALL

 choice

     bool

     prompt "Kernel verbosity:"

     default KERNEL_LINUX_VERBOSITY_0

 config KERNEL_LINUX_VERBOSITY_0

     bool

     prompt "Simplified"

     help

       Print simplified command lines.

 config KERNEL_LINUX_VERBOSITY_1

     bool

     prompt "Full commands"

     help

       Print full command lines.

 config KERNEL_LINUX_VERBOSITY_2

     bool

     prompt "Exec reasons"

     help

       Print the reasons why a make target is rebuild.

 endchoice

 config KERNEL_LINUX_VERBOSE_LEVEL

     int

     default 0 if KERNEL_LINUX_VERBOSITY_0

     default 1 if KERNEL_LINUX_VERBOSITY_1

     default 2 if KERNEL_LINUX_VERBOSITY_2

 config KERNEL_LINUX_INSTALL_CHECK

     bool

     prompt "Check installed headers"

     default y

     help

       If you are in doubt that installed headers are buggy, say 'Y'

       here to have an extra check passed onto the headers.

 endif # KERNEL_LINUX_INSTALL