toolchain: reduce the list of locales to keep by default

Our current list of locales to keep by default is "C en_US de fr". It
doesn't make much sense to keep "de" and "fr" more than any other
language. So let's keep only the "C" and "en_US" locales by default,
and leave it to the user to specify other locales to keep if needed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>