api: Use BuildPrebuiltsRequest.build_target

https://crrev.com/c/4136874 added a new field "build_target" to
SdkService.BuildPrebuiltsRequest. This CL forwards the passed-in value
to scripts/build_sdk_board, as --board.

BUG=b:250023328
TEST=Invoke BAPI endpoint with build_target, ensure script is called
with --board
TEST=Invoke BAPI endpoint without build_target, ensure script is called
with ordinary default
TEST=./run_tests

Change-Id: Ibbe28d4ddeed6ec039f5b51a0d0d68bd32ea132f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4136955
Commit-Queue: Greg Edelston <gredelston@google.com>
Tested-by: Greg Edelston <gredelston@google.com>
Reviewed-by: Lizzy Presland <zland@google.com>
diff --git a/api/controller/sdk.py b/api/controller/sdk.py
index 200d55d..cde767f 100644
--- a/api/controller/sdk.py
+++ b/api/controller/sdk.py
@@ -175,7 +175,7 @@
 def BuildPrebuilts(input_proto, _output_proto, _config):
     """Builds the binary packages that comprise the Chromium OS SDK."""
     chroot = controller_util.ParseChroot(input_proto.chroot)
-    sdk.BuildPrebuilts(chroot)
+    sdk.BuildPrebuilts(chroot, board=input_proto.build_target.name)
 
 
 @faux.success(_CLUris)