pptp-linux: add from OE-Classic, update and tidy up

* Update to 1.7.2
* Drop unnecessary override of do_compile
* Make LICENSE more accurate
* Add LIC_FILES_CHKSUM
* Add SUMMARY
* Minor formatting/ordering tweaks

Based on a patch by Vladimir Redzhepoff <vladimir.redzhepoff@promwad.com>

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>