[1/2] P51XX: move TWRP config to common

Use BoardCommonConfig.mk for main TWRP config.
Use p51xx-common.mk to place twrp.fstab inside TWRP.

Change-Id: I1bdd1f30afbcca06dec45184b63e299bece2cc9d
P5100: Cleanup

1. Move "DEVICE_RESOLUTION" to common
2. move twrp.fstab inside rootdir
3. get rid of modem line in twrp.fstab
4. Remove unneeded "PRODUCT_PACKAGES"

Change-Id: Ib66ae93bd4bc709b56416c8a7582f280e8d45573