kernel: add mingw

Add the option to build a cross-compiler for kernel type 'mingw'.
The resulting cross-compiler can be used to build applications on a Linux host
that can be run on a Windows target.

Compiler is build using the mingwrt and w32-api packages aviable from the
MinGW project (http://sourceforge.net/projects/mingw).

The windows headers (w32-api package) are extracting with the kernel_headers
step The libraries and other headers from both packages are build and
installed in the various steps of libc

Signed-off-by: Bart vdr Meulen <bartvdrmeulen@gmail.com>
[yann.morin.1998@anciens.enib.fr: fix kernel headers comment, don't "return 0"]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# Compute IA-64-specific values

CT_DoArchTupleValues() {

    # The architecture part of the tuple, override only for 64-bit

    if [ "${CT_ARCH_64}" = "y" ]; then

        CT_TARGET_ARCH="mips64${target_endian_el}"

    else

        # The architecture part of the tuple:

        CT_TARGET_ARCH="${CT_ARCH}${target_endian_el}"

    fi

    # Override CFLAGS for endianness:

    case "${CT_ARCH_BE},${CT_ARCH_LE}" in

        y,) CT_ARCH_ENDIAN_CFLAG="-EB";;

        ,y) CT_ARCH_ENDIAN_CFLAG="-EL";;

    esac

}