Makefile: do not purge locale-archive from target

The rules to purge unwanted locales from the target also removed
the locale-archive file from /usr/lib/locale which contains the
locale definitions explicitly generated for the target.

Signed-off-by: Sven Neumann <neumann@teufel.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>