WIP Chromite supporting "LEGACY" mode, as requested by Anush.

chromite/specs/build/x86-agz.spec

+# chromite build spec file
+# Use RFC 822 format
+
+[LEGACY]
+
+# The board name that will be passed to various commands.
+#
+# DEFAULT: the name of this spec file, without the suffix
+#board: x86-agz
+
+# A spec file describing how to build/enter the chroot.  Can be an absolute
+# path or the exact name of a spec file (without the suffix) in the chroot
+# spec search path.
+#
+# DEFAULT: chroot
+#chroot_spec: chroot
+
+
+[LEGACY_BUILD]
+
+# Flags to pass to setup_board (other than --board).  Note that setup_board is
+# only called once per board, unless you use "chromite clean" to clean your
+# board out.
+#
+# DEFAULT: nothing
+#setup_board_flags:
+
+# Flags to pass to build_packages (other than --board).
+#
+# DEFAULT: nothing
+build_packages_flags: --oldchromebinary
+
+
+[LEGACY_IMAGE]
+
+# Flags to pass to build_image (other than --board)
+#
+# DEFAULT: nothing
+#build_image_flags: