ppp: rework host-uniq support to take hex encoded strings

The previous implementation of the "host-uniq" option used plain strings for
passing the value to pppd which made it impossible to specify binary data.

Switch the format to a hex encoded string to support binary data.

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

SVN-Revision: 44094