Teach fastboot to allow required variables per product.

This is needed for products like xoom-cdma and xoom-cdma-lte.
The xoom-cdma-lte product requires an lte baseband binary but
it's not needed for xoom-cdma.

This is implemented by allowing an optional product parameter
to "required" statements. The parameter is separated from
"required" by a colon so the version-baseband-2 requirment in
board-info.txt for stingray becomes:

 require-for-product:xoom-cdma-lte version-baseband-2=ltedc_u_03.25.00|ltedc_u_03.19.00

In the above statement, only xoom-cdma-lte requires version-baseband-2
and the baseband can be lte_u_03.25.00 or lte_u_03.19.00. For other
products version-baseband-2 will be ignored.


Change-Id: I786bec5f5661c2243d87925b064fc6124d3cffa1