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>