board: toradex: Quote variables in `test` cmd expression

board: toradex: Quote variables in `test` cmd expression

With correct POSIX handling, unquoted empty variables can turn the
expression like
	test -n ${fdtfile}
into
	test -n

The POSIX handling for single argument `test` evaluates it as true,
so the fallback initialization will be skipped unexpectedly.
Quoting variable expansions in `test` expressions will always result in
correct behavior for empty and non-empty values.
This change was triggered by
commit 8b0619579b22 ("cmd: test: fix handling of single-argument form of test")
The aim is to have a less fragile codebase that is not dependent on a
quirk of the shell implementation.

Use quoted variable expansions in `test` expressions throughout.

Signed-off-by: Franz Schnyder <franz.schnyder@toradex.com>
Acked-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Simon Glass <sjg@chromium.org>