minui: Allow skipping EV_REL input devices.

This is causing recovery to skip real input devices on some
samsung phones where sensors are registered as input devices.
So there more then 16 of them. (ex. ks01lte)
And EV_REL input devices already disabled in recovery ui.cpp if
BOARD_RECOVERY_NEEDS_REL_INPUT is not set. So do same here not to exceed
the limit of MAX_DEVICES

[javelinanddart]: Move this to an opt-in flag so that we can document
  what devices need this in the device tree, since otherwise we'll
  forget to port this and what devices need this flag. Additionally,
  an opt-in flag makes more sense for select devices needing
  non-standard behavior. The flag is used as follows:
  TARGET_RECOVERY_SKIP_EV_REL_INPUT := true

Change-Id: If3d6e29d00229278a8ef3dfa445393c9f3d5f361
(cherry picked from commit 331afccf)
(cherry picked from commit e7597e93)