Set Google DNS on MK1 SIM900

Adds the command during Cell Modem configuration to set the DNS
to the Google Public 8.8.8.8 and 8.8.4.4 DNS servers.

Untested, need someone with working MK1 + Telemetry or MK2 to test.

Conflicts:
	src/devices/cellModem.c
Fix bug in strncmp usage in cellModem.c

The value was too damn small to compare against.  Thus it could
hit as true, even though it may not be actually true (since you
don't validate all the characters).  This simply fixes that bug.