p3100: Device makefile cleanup

-Removes the redundant full_p3100
-Standardizes the layout of omni_p3100
-Align ro.product with stock
-Updates the fingerprint to P3100XWDNA1 (4.2.2)

  PS2: Add README
  PS3: ro.product.device is actually set with TARGET_DEVICE & not
       PRODUCT_DEVICE; this has been corrected.
  PS4: PRODUCT_RELEASE_NAME isn't used in omnirom.
  PS5: Align this change to the changes I'm making in the other
       device repos
  PS6: config_enableScreenrecordChord is already set to true by
       default

Change-Id: Iaad0fbf7cc37c1321c99bc47579d2d7035db2609