toolchain: Clean up locale support menu

Closes #421.

This patch cleans up and fixes some minor issues with the locale support
section of the toolchain menu.

1. uClibc requires wchar support if locales are enabled, make locale
   support select wchar support.

2.  Allow purging of locale information even if there is no locale
    support in the C library.  This cleans up after packages that
    install things into /usr/share/locale on the target.

Signed-off-by: Will Newton <will.newton@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>