wear_ui: Expose menu_unusable_rows via Makefile var.

This variable is useful on small screens (e.g. on watches) to handle
long menus. We should have better way to handle this value smartly.
Prior to that, expose the value to be overridable by using the generic
wearable UI module (librecovery_ui_wear).

Bug: 64307776
Test: Define the variable, build and boot into recovery image and check
      the UI menu.
Change-Id: I5d7a6baa8bb4cc852bfcc2a7b3cc9686c1c8817e