Add a product for building linux_bionic in --soong-only mode.

Modelled after the mainline_sdk product.

Test: build/soong/soong_ui.bash --make-mode --soong-only \
        SOONG_ALLOW_MISSING_DEPENDENCIES=true \
        TARGET_PRODUCT=linux_bionic com.android.art.host
Bug: 31559095
Bug: 229123823
Change-Id: I6051d21cc4bae561e987561ddad5f1ec5435c4ca
diff --git a/target/board/linux_bionic/BoardConfig.mk b/target/board/linux_bionic/BoardConfig.mk
new file mode 100644
index 0000000..7938bdb
--- /dev/null
+++ b/target/board/linux_bionic/BoardConfig.mk
@@ -0,0 +1,24 @@
+# Copyright (C) 2020 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+# This "device" is only intended to be used for host Bionic build targets, so
+# (device) target architectures are irrelevant. However, the build system isn't
+# prepared to handle no target architectures at all, so pick something
+# arbitrarily.
+TARGET_ARCH_SUITE := ndk
+
+HOST_CROSS_OS := linux_bionic
+HOST_CROSS_ARCH := x86_64
+HOST_CROSS_2ND_ARCH :=