Fix render issue when button has conditional

If the button has a conditional, the image render function will
return a 0 which indicated that an image was present even if it
was not if the condition was not being met during boot. When the
condition is met, mButtonImg was not set to NULL causing a render
request failure.

Change-Id: I32be0ddcdc1bf7ad22440d66018ef09484280afa
Support custom graphics.c.

TWRP does not use this, but is needed for other things that depend
on it like charger.

Change-Id: Ic7d10e8230151bcc4be6cfa222a1e07231e7139f
Merge pull request #1 from omnirom/android-4.4

Merge from omni