AGESA: Introduce AGESA_LEGACY and its counterpart

We define AGESA_LEGACY as an implementation of mainboard
that has its romstage main completely under mainboard/
directory. We have learnt from other platforms this approach
has several downsides when it comes to making platform-wide
improvements.

We start by creating per-family romstage.c file, which
boards will gradually take into use by removing the
AGESA_LEGACY Kconfig option we here apply to all of them.

Change-Id: Id01931e185a023039a60af16a678de9966db8d65
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/18619
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marc@marcjonesconsulting.com>
Reviewed-by: Martin Roth <martinroth@google.com>

src/mainboard/biostar/am1ml/Kconfig

@@ -19,6 +19,7 @@ if BOARD_BIOSTAR_AM1ML
 config BOARD_SPECIFIC_OPTIONS
 	def_bool y
 	select BOARD_ROMSIZE_KB_4096
+	select AGESA_LEGACY
 	select CPU_AMD_AGESA_FAMILY16_KB
 	select FORCE_AM1_SOCKET_SUPPORT
 	select GFXUMA