Introduce b into envsetup.sh.

`b` is a function that combines:

1. Integrated generation of a synthetic bazel workspace, containing
symlinks to BUILD and bzl files alongside symlinks to the source tree.
This is the --package_path of the bazel build.
2. Running the Bazel build itself with b's entire argv.

A user accustomed to typing `bazel build <targets>` would
now type `b build <targets>`.

Test: source build/envsetup.sh; b build //bionic/...; b cquery --output=label_kind //bionic/...
Fixes: 188490434
Change-Id: I36e366108b024c09945d764a1115786658e03681

envsetup.sh

@@ -1687,10 +1687,19 @@ function _trigger_build()
     if T="$(gettop)"; then
       _wrap_build "$T/build/soong/soong_ui.bash" --build-mode --${bc} --dir="$(pwd)" "$@"
     else
-      echo "Couldn't locate the top of the tree. Try setting TOP."
+      >&2 echo "Couldn't locate the top of the tree. Try setting TOP."
+      return 1
     fi
 )
 
+function b()
+(
+    # Generate BUILD, bzl files into the synthetic Bazel workspace (out/soong/workspace).
+    m nothing GENERATE_BAZEL_FILES=true || return 1
+    # Then, run Bazel using the synthetic workspace as the --package_path.
+    "$(gettop)/tools/bazel" "$@" --config=bp2build
+)
+
 function m()
 (
     _trigger_build "all-modules" "$@"