hwdb: strip the root from filenames when generating hwdb.bin

The modern hwdb.bin format contains the filenames of the input data that
makes up the database.  This is useful but in offline builds where
--root is used, the filenames are the full build paths including the
specified root.  This introduces build paths and thus information
leakage and non-reproducible data.

Solve this by stripping the root prefix off the original path when
passing to import_file.

Add TEST-17-UDEV.hwdb.sh to verify that hwdb.bin files contain the
path inside the root, but not the path of the root.

test/units/TEST-17-UDEV.hwdb.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+set -eux
+set -o pipefail
+
+if ! command -v systemd-hwdb >/dev/null; then
+    echo "systemd-hwdb not found, skipping."
+    exit 0
+fi
+
+# shellcheck source=test/units/util.sh
+. "$(dirname "$0")"/util.sh
+
+at_exit() (
+    set +e
+
+    [[ -d "$ROOTFS" ]] && rm -rf "$ROOTFS"
+)
+trap at_exit EXIT
+
+ROOTFS="$(mktemp -d -t test-17-udev-hwdb.XXXX)"
+HWDB=$ROOTFS/usr/lib/udev/hwdb.d
+
+mkdir -p "$HWDB"
+cat >"$HWDB/99-test.hwdb" <<EOF
+scsi:*
+  ID_TEST=test
+EOF
+
+systemd-hwdb update --root "$ROOTFS" --usr
+
+# check the rootfs path does not appear in hwdb.bin
+run_and_grep -n "$ROOTFS" strings "$ROOTFS/usr/lib/udev/hwdb.bin"
+# check the path in the rootfs does appear in hwdb.bin
+run_and_grep 99-test.hwdb strings "$ROOTFS/usr/lib/udev/hwdb.bin"