Pointing common.mk's SOURCE_DIR to subdirectories of fpga, to avoid circular dependency caused by pointing to fpga, which contains generated-src.
Pointing common.mk's SOURCE_DIR to subdirectories of fpga, to avoid circular dependency caused by pointing to fpga, which contains generated-src.

ucb-bar/chipyard,ucb-bar/chipyard,ucb-bar/chipyard,ucb-bar/chipyard