mccmon6: Quote recovery_status in bootcmd test

The mccmon6 bootcmd starts with:

    if test -n ${recovery_status}; then run boot_recovery; ...

The default state is "no recovery requested", i.e. recovery_status
unset.  The expression then expands to 'test -n' with no operand and
relies on a U-Boot 'test' quirk that treats a missing operand as
false to skip recovery.

Quote the variable so an unset recovery_status expands to 'test -n ""'
and the emptiness check is explicit.

Fixes: 8b0619579b ("cmd: test: fix handling of single-argument form of test")
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Łukasz Majewski <lukma@nabladev.com>

include/configs/mccmon6.h

@@ -88,7 +88,7 @@
 		     "bootm $loadaddr};reset;" \
 		"fi\0" \
 	"bootcmd=" \
-		"if test -n ${recovery_status}; then " \
+		"if test -n \"${recovery_status}\"; then " \
 		     "run boot_recovery;" \
 		"else " \
 		     "if test ! -n ${boot_medium}; then " \