hlr: implement str2apn() to convert an apn as string into octetstring

APN are encoded by splitting each domain part by the dot and prefix
each element by a 8bit length.
E.g. internet -> \x08internet and internet.foo -> \x08internet\x03foo

Change-Id: I607969cd58110d4d5ff1b828e64cf2b5031868ac
1 file changed