Remove usage of intermediates-dir-for

intermediates-dir-for relies on several variables that
haven't been set at the time the board configuration
runs. The board configuration got around that by using
deferred expansion, but deferred expansion is something
that starlark doesn't support. Remove intermediates-dir-for
by switching to TARGET_RECOVERY_FSTAB_GENRULE, which
accepts a genrule module to use instead of a file.

Bug: 201700692
Test: Presubmits
Change-Id: I6ebb75073abeebce71873f414129759789096b0f
diff --git a/BoardConfig-common.mk b/BoardConfig-common.mk
index b065cf7..f775fcc 100644
--- a/BoardConfig-common.mk
+++ b/BoardConfig-common.mk
@@ -68,12 +68,7 @@
 # This is the fstab file that will be included in the recovery image.  Note that
 # recovery doesn't care about the encryption settings, so it doesn't matter
 # whether we use the normal or the fips fstab here.
-#
-# Since this is a generated file, it's necessary to use intermediates-dir-for in
-# order to refer to it correctly.  And since intermediates-dir-for isn't defined
-# yet when this file is included, it's necessary to use a deferred variable
-# assignment ( = ) rather than an immediate variable assignment ( := ).
-TARGET_RECOVERY_FSTAB = $(call intermediates-dir-for,ETC,fstab.gs101)/fstab.gs101
+TARGET_RECOVERY_FSTAB_GENRULE = gen_fstab.gs101
 
 TARGET_RECOVERY_PIXEL_FORMAT := ABGR_8888
 TARGET_RECOVERY_UI_MARGIN_HEIGHT := 165