Change AMD vendorcode build

Change AMD vendorcode build

Apply the normal method of recursively including subdirectories
for src/vendorcode. Remove redundant references under
mainboard and northbridge.

Change-Id: I914a6e262ed2abe83f407df36fe5c1af5eb4bcb0
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/468
Tested-by: build bot (Jenkins)
Reviewed-by: Kerry Sheh <shekairui@gmail.com>
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>