Some common workflows for developing with buffet:

# Tell portage that you'd like to make local changes to Buffet:
cros_workon start --board=${BOARD} platform2

# Edit files in platform2/buffet/
vim ...

# Compile and install those changes into the chroot:
USE=buffet emerge-<board> platform2

# Compile and run buffet unittests
USE=buffet P2_TEST_FILTER="buffet::*" FEATURES=test emerge-<board> platform2

# Deploy the most recently built version of buffet to a DUT:
cros deploy --board=${BOARD} <remote host> platform2