product_config: Set CARBON_BUILD priority above TARGET_BUILD_APPS

Even if we are doing TARGET_BUILD_APPS, we may want to use a
carbon target for building.

Change-Id: Ic8053bc679fa8b726be90e611aeaacdf1850cabc
Merge pull request #25 from CarbonDev/kk

product_config: Set CARBON_BUILD priority above TARGET_BUILD_APPS