rootfs: isolate build workspace from source tree

Problem
- ROOTFS_DIR was bound to $WORKDIR/rootfs while the script itself
  resides under rootfs/scripts/ in the same repository.
- The preprocessing stage performs `rm -rf "$ROOTFS_DIR"`; when invoked
  from qcom-build-utils/, that path overlaps the source tree and can
  remove the script directory used by subsequent invocations.

Root cause
- The working rootfs output directory and the repository source
  directory shared the same basename (rootfs) and filesystem root.

Implementation
- Rename the runtime extraction/build directory from $WORKDIR/rootfs
  to $WORKDIR/rootfs_work.

Operational impact
- Prevents destructive collision between generated rootfs state and
  checked-in script sources.
- Enables repeated invocations in a single CI job (e.g. multi-kernel
  loops) without losing rootfs/scripts/build-rootfs.sh mid-run.

Scope
- Path-safety fix only; no package-selection or install-order behavior
  changes.

Signed-off-by: Bjordis Collaku <bcollaku@qti.qualcomm.com>

Author: bjordiscollaku

rootfs/scripts/build-rootfs.sh

@@ -139,7 +139,7 @@ fi
 
 WORKDIR=$(pwd)
 MNT_DIR="$WORKDIR/mnt"
-ROOTFS_DIR="$WORKDIR/rootfs"
+ROOTFS_DIR="$WORKDIR/rootfs_work"
 ROOTFS_IMG="rootfs.img"
 mkdir -p "$MNT_DIR" "$ROOTFS_DIR"