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>

 # mingw options

 # depends on MINGW32

 config LIBC_mingw

     bool

     select LIBC_SUPPORT_WIN32THREADS

     help

       The de-facto standard for Mingw distributions.

 choice

     bool

     prompt "Mingw runtime version"

 # Don't remove next line

 # CT_INSERT_VERSION_BELOW

 config MINGWRT_V_3_18

     bool

     prompt "3.18"

 config MINGWRT_V_select

     bool

     prompt "Other version"

 endchoice

 config MINGWRT_VERSION

     string

     prompt "Mingw runtime version" if MINGWRT_V_select

 # Don't remove next line

 # CT_INSERT_VERSION_STRING_BELOW

     default "3.18" if MINGWRT_V_3_18

     help

       Enter the version number of the mingw runtime files to use