[external build systems] scripts/configure: allow out-of-tree project root (project-chip#29627)

The configure script, intended for external build systems such as openwrt,
so far only allowed projects contained within the connectedhomeip source tree.

This is the case for CHIP examples, but is NOT viable for actual applications,
where connectedhomeip is usually a submodule of the main project.

This PR changes `scripts/configure` in a backwards compatible way as follows:

- absolute paths specified with `--project` are now supported
- relative paths specified with `--project` are still relative to CHIP_ROOT, but
  are also allowed to point outside (using ../../... type path).

Tested with actual build of p44mbrd (https://github.com/plan44/p44mbrd)
as openwrt package (https://github.com/plan44/plan44-feed/tree/main/p44mbrd)

Author: plan44

scripts/configure

@@ -41,7 +41,8 @@ function usage() { # status
     info "Usage: $0 [OPTIONS] [--project=... [PROJECT OPTIONS]]"
     info "Options:"
     info "  --build-env-dir=DIR   Directory to create (host) build environment in"
-    info "  --project=DIR         Sub-directory to build, eg examples/lighting-app/linux"
+    info "  --project=DIR         directory to build, absolute or relative to chip root,"
+    info "                        eg examples/lighting-app/linux or /my/dir/my/app"
     info ""
     info "Project options (mapped to GN build args):"
     info "  --enable-<ARG>[=no]   Enables (or disables with '=no') a bool build arg"
@@ -68,6 +69,7 @@ function main() { # ...
     # Parse global options, process VAR=VALUE style arguments, and collect project options
     BUILD_ENV_DIR=
     PROJECT=
+    PROJECT_PATH=
     PROJECT_ARGS=()
     while [[ $# -gt 0 ]]; do
         case "$1" in
@@ -87,10 +89,8 @@ function main() { # ...
     [[ -n "$PROJECT" || -n "$BUILD_ENV_DIR" ]] || usage 1
 
     if [[ -n "$PROJECT" ]]; then
-        local subdir="$(cd "${CHIP_ROOT}/${PROJECT}" 2>/dev/null && pwd)"
-        [[ -n "$subdir" && -r "${subdir}/.gn" ]] || fail "Invalid project '${PROJECT}'"
-        PROJECT="${subdir#${CHIP_ROOT}/}"
-        [[ "$subdir" == "${CHIP_ROOT}/${PROJECT}" ]] || fail "Unable to determine project path"
+        PROJECT_PATH="$(cd "${CHIP_ROOT}" 2>/dev/null && cd "${PROJECT}" 2>/dev/null && pwd)"
+        [[ -n "$PROJECT_PATH" && -r "${PROJECT_PATH}/.gn" ]] || fail "Invalid project '${PROJECT}' - missing .gn at '${PROJECT_PATH}'"
     fi
 
     check_binary gn GN
@@ -177,13 +177,13 @@ function gn_generate() { # [project options]
     ensure_no_clobber "${BUILD_DIR}/args.gn"
 
     # Pass --script-executable to all `gn` calls so scripts run in our venv
-    local gn=(gn --script-executable="${BUILD_ENV_DIR}/bin/python" --root="${CHIP_ROOT}/${PROJECT}")
+    local gn=(gn --script-executable="${BUILD_ENV_DIR}/bin/python" --root="${PROJECT_PATH}")
 
     # Run gn gen with an empty args.gn first so we can list all arguments
     info "Configuring gn build arguments (see $BUILD_DIR/args.configured for full list)"
     {
         echo "# ${CONFIGURE_MARKER}"
-        echo "# project root: ${PROJECT}"
+        echo "# project root: ${PROJECT_PATH}"
     } >"${BUILD_DIR}/args.gn"
     "${gn[@]}" -q gen "$BUILD_DIR"