Build - Add PRODUCT_BOOTANIMATION

Just add PRODUCT_BOOTANIMATION in your product
makefiles and point it to the zip file you would
like to have used as the boot animation.

The coresponsing build system handles picking
the last one, which is the last product to set this.

Signed-off-by: William Roberts <wroberts@tresys.com>

Fix for build break when PRODUCT_BOOTANIMATION is unset

Restore original behavior when PRODUCT_BOOTANIMATION is undefined

Change-Id: I7ec578fee65d3cbef5b0d22ae4b413131e6c9589
Signed-off-by: STELIX <ssspinni@gmail.com>