config: Set some default TWRP flags

When WITH_TWRP=true is set:
* Set recovery variant to TWRP, it should be synced
  under bootable/recovery-twrp in Lineage
* Exclude SuperSU, we have our own su solution

Change-Id: I5e5e6f2b42282c4666cdc013405a1488a9c6341d