env import/export: Remove from help if disabled

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Wolfgang Denk <wd@denx.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>

common/cmd_nvedit.c

@@ -954,11 +954,15 @@ U_BOOT_CMD(
 #if defined(CONFIG_CMD_EDITENV)
 	"env edit name - edit environment variable\n"
 #endif
+#if defined(CONFIG_CMD_EXPORTENV)
 	"env export [-t | -b | -c] [-s size] addr [var ...] - export environment\n"
+#endif
 #if defined(CONFIG_CMD_GREPENV)
 	"env grep string [...] - search environment\n"
 #endif
+#if defined(CONFIG_CMD_IMPORTENV)
 	"env import [-d] [-t | -b | -c] addr [size] - import environment\n"
+#endif
 	"env print [name ...] - print environment\n"
 #if defined(CONFIG_CMD_RUN)
 	"env run var [...] - run commands in an environment variable\n"