glibc, local.conf.sample: Introduce and handle new OE variable, GLIBC_GENERATE_LOCALES

glibc, local.conf.sample: Introduce and handle new OE variable, GLIBC_GENERATE_LOCALES
to limit set of generated binary locales.
* If ENABLE_BINARY_LOCALE_GENERATION is set to "1", you can limit locales
generated to the list provided by GLIBC_GENERATE_LOCALES. This is huge
time-savior for developmental builds. Format: list of locale.encoding pairs
with spaces as separators, e.g.:
GLIBC_GENERATE_LOCALES = "en_GB.UTF-8 de_DE.UTF-8"
* Check your distro for minimal set of locales required. Old-time default is
"en_GB.UTF-8 de_DE.UTF-8 fr_FR.UTF-8".
* Angstrom requires en_GB only.
* This commit does not changes the default, which is still generation of all
available locales.
* Closes #1966.