lib: Create a new Kconfig option for charset conversion

Rather than looking at two KConfig options in the Makefile, create a new
Kconfig option for compiling lib/charset.c

Enable it for UFS also, which needs this support.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>