Make mksquashfs generate a fs_config-friendly root entry

This change adds a flag so that the mksquashfs wrapper scripts generate
an entry that is fs_config-friendly for the root inode. Namely, the root
entry is expected to have an empty filename.

Bug: 72745016
Test: m -j100 dist
Change-Id: Iebdf79c5af0b9d999b7e5f5fe240abfe52cbadda
diff --git a/core/Makefile b/core/Makefile
index a3fbe33..9a582c3 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -2342,7 +2342,7 @@
 # $1: root directory
 # $2: add prefix
 define fs_config
-(cd $(1); find . -type d | sed 's,$$,/,'; find . \! -type d) | cut -c 3- | sort | sed 's,^,$(2),' | $(HOST_OUT_EXECUTABLES)/fs_config -C -D $(TARGET_OUT) -S $(SELINUX_FC)
+(cd $(1); find . -type d | sed 's,$$,/,'; find . \! -type d) | cut -c 3- | sort | sed 's,^,$(2),' | $(HOST_OUT_EXECUTABLES)/fs_config -C -D $(TARGET_OUT) -S $(SELINUX_FC) -R "$(2)"
 endef
 
 # Depending on the various images guarantees that the underlying