imx: hab: Improve the hab_auth_img_or_fail usage text

Split the hab_auth_img_or_fail usage text in two sentences to make it
clearer.

Signed-off-by: Fabio Estevam <festevam@denx.de>
diff --git a/arch/arm/mach-imx/hab.c b/arch/arm/mach-imx/hab.c
index 8ba3c5f..e7b0388 100644
--- a/arch/arm/mach-imx/hab.c
+++ b/arch/arm/mach-imx/hab.c
@@ -715,7 +715,7 @@
 U_BOOT_CMD(
 		hab_auth_img_or_fail, 4, 0,
 		do_authenticate_image_or_failover,
-		"authenticate image via HAB on failure drop to USB BootROM mode",
+		"authenticate image via HAB. Switch to USB BootROM mode on failure",
 		"addr length ivt_offset\n"
 		"addr - image hex address\n"
 		"length - image hex length\n"