gbmc-no-initrd: Fix shellcheck SC2157 error

The check [ -n '@ALWAYS_CLEAR@' ] becomes [ -n '1' ] when substituted,
which shellcheck flags as always true.

Change it to a value comparison [ '@ALWAYS_CLEAR@' = '1' ] which
satisfies shellcheck and maintains the same logic.

no fusion2 link for preEVT
Tested:
```
 MACHINE=platform28 GBMC_CONFIG=dev bitbake gbmc-no-initrd
 // pass
```

Google-Bug-Id: n/a
Change-Id: I556deaadcf55bc983290746dd4bc1bf543e0a279
Signed-off-by: Tom Tung <tomtung@google.com>

recipes-google/boot/gbmc-no-initrd/clean-rwfs.sh.in

diff --git a/recipes-google/boot/gbmc-no-initrd/clean-rwfs.sh.in b/recipes-google/boot/gbmc-no-initrd/clean-rwfs.sh.in
index 4fc36f3..04084e5 100644
--- a/recipes-google/boot/gbmc-no-initrd/clean-rwfs.sh.in
+++ b/recipes-google/boot/gbmc-no-initrd/clean-rwfs.sh.in
@@ -39,7 +39,7 @@
   mount_rwfs || ret=$?
   mkdir -p "$(dirname "$done_job_id")"
   cp -v "$job_id_tmp" "$done_job_id"
-elif [ -n '@ALWAYS_CLEAR@' ] || ! cmp "$rwfs"/os-release /etc/os-release 2>/dev/null; then
+elif [ '@ALWAYS_CLEAR@' = '1' ] || ! cmp "$rwfs"/os-release /etc/os-release 2>/dev/null; then
   declare -A allowed=()
   while read -r line; do
     allowed["$line/*"]=1