menuconfig:
 * add support for override options for download toolchains
 * refs #1939
 * refs #1999

From now on one can build his own toolchains and use them as download toolchains by overriding the corresponding options under "Override options/Override precompiled toolchain options":
 1. activate "Toolchain options/Build own toolchains"
 2. set toolchain related options to the desired ones under "Toolchain options"
 3. (optional) make your own modifications under $(freetz_root)/toolchain
 4. call "make KTV=freetz-${MY_VERSION}-${MY_SUFFIX} TTV=freetz-${MY_VERSION}-${MY_SUFFIX} toolchain"
 5. wait the build to complete
 6. (optional) upload created download toolchain files to some site

The toolchains created in steps above can then be reused:
 7. activate "Toolchain options/Download and use precompiled toolchains"
 8. activate "Override options/Override precompiled toolchain options"
 9. set version/suffix/md5/download-site values to the values used in the steps above
10. adjust gcc/uClibc versions under "Toolchain options", set them to the same values as in step 2


git-svn-id: file:///var/svn/freetz/trunk@9983 149334a1-2f27-0410-a3b9-fc62619ac1e6