Add new user setup command banchan for module building.

It currently uses TARGET_BUILD_APPS just like tapas, but the use case is
different and it may diverge more in the future.

Test: banchan com.android.art
Test: banchan help
Test: banchan
Test: hmm
Bug: 179779520
Change-Id: Iae718e65a2a7212c741c397e03c6f9a6d5ee8951

banchanHelp.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+
+# locate some directories
+cd "$(dirname $0)"
+SCRIPT_DIR="${PWD}"
+cd ../..
+TOP="${PWD}"
+
+message='usage: banchan <module> ... [arm|x86|arm64|x86_64] [eng|userdebug|user]
+
+banchan selects individual APEX modules to be built by the Android build system.
+Like "tapas", "banchan" does not request the building of images for a device but
+instead configures it for an unbundled build of the given modules, suitable for
+installing on any api-compatible device.
+
+The difference from "tapas" is that "banchan" sets the appropriate products etc
+for building APEX modules rather than apps (APKs).
+
+The module names should match apex{} modules in Android.bp files, typically
+starting with "com.android.".
+
+The usage of the other arguments matches that of the rest of the platform
+build system and can be found by running `m help`'
+
+echo "$message"