Makefile: add BR2_ENABLE_LOCALE_PURGE to purge unwanted locales

Add BR2_ENABLE_LOCALE_PURGE / BR2_ENABLE_LOCALE_WHITELIST options to
remove unwanted locales from the target rootfs. Handy for stuff like
the gtk stack, which comes with ~25 MB locales.

Works similar to localepurge in Debian, E.G. you provide a white list
of wanted locales, and everything else is removed.