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.

Change-Id: I8c95a515a8fbb92d363141eb79e254712dccc162
Signed-off-by: William Roberts <wroberts@tresys.com>
Fix for build break when PRODUCT_BOOTANIMATION is unset

Change-Id: I236c2dd35ba0e632ed327ed6dc36324c9e59587a