Fix: Use half-resolution version of the boot animation for better performance

This needs to be defined in cm.mk or it has no effect.
BoardConfig.mk seems not to be picked up by vendor/cm/config/common.mk

[steven@steven676.net: remove useless flag from BoardConfig.mk too]

Change-Id: I69de3e1d17b2404104d2e0f132367733092f39cd