scripts: leave changelog in build dir, copy to install dir

Users tend to look for the build log in the current working directory,
rather than in the toolchain's installation dir. While bundling the build
log in the toolchain installation dir is nice for distribution and review,
it can be easier to have the build log readily available in the working
directory, as it is quicker to get to it.

So, the build log stays in the working directory until the toolchain is
completely and successfully built, and then a (compressed) copy is made.

Reported-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # x86 specific options

 config ARCH_x86

     select ARCH_SUPPORTS_32

     select ARCH_SUPPORTS_64

     select ARCH_DEFAULT_32

     select ARCH_USE_MMU

     select ARCH_SUPPORT_ARCH

     select ARCH_SUPPORT_CPU

     select ARCH_SUPPORT_TUNE

     help

       The x86 architecture, as defined by:

         32-bit (ia32) : http://www.intel.com/

         64-bit (amd64): http://www.amd.com/us-en/Processors/DevelopWithAMD/0,,30_2252_875_7044,00.html