busybox: make high ASCII chars printable (#7993)

Currently busybox utils like "ls" fail to display filenames containing UTF-8
characters, replacing any special characters with "?".

Change libbb's printable_string() function to allow high ASCII characters so
that unicode filenames are displayed correctls.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 43084