Add new libisi recipe (libisi can talk to the nokia900 modem)

Add new libisi recipe (libisi can talk to the nokia900 modem)

libisi will be used in fsogsmd for the nokia900.
As we can't use ofono directly we import some ofono parts in libisi,
  and use that library in the vala code of fsogsmd trough a vapi file.

Thanks a lot mickeyl for help on #openmoko-cdevel on Freenode on irc:
  <mickeyl> do you use =+ or += ?
  <mickeyl> PN-tests probably need to be prepended to PACKAGES, not appended
(I was doing PACKAGES += "${PN}-test" and it failed to package the tests)

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>