[travis] Integrate with Coverity Scan

Signed-off-by: Michael Brown <mcb30@ipxe.org>

.travis.yml

       - liblzma-dev
       - syslinux
       - genisoimage
+  coverity_scan:
+    project:
+      name: "ipxe/ipxe"
+      version: $TRAVIS_COMMIT
+    build_command_prepend: "make -C src bin/deps"
+    build_command: "make -C src bin/blib.a"
+    branch_pattern: coverity_scan
 
 env:
   global: