aboot: Restore Android boot image check for non-lk2nd configs

Non-Android boot images can only be booted when the extlinux lk2nd/boot
module is enabled, so let's restore the check for configurations without
that module.

This reverts parts of commit 6a2af94.

app/aboot/aboot.c

@@ -3943,6 +3943,12 @@ void cmd_flash_mmc_img(const char *arg, void *data, unsigned sz)
 			if (!strncmp(pname, "boot", strlen("boot"))
 					|| !strcmp(pname, "recovery"))
 			{
+				if (!IS_ENABLED(WITH_LK2ND_BOOT) &&
+				    ((sz < BOOT_MAGIC_SIZE) || memcmp((void *)data, BOOT_MAGIC, BOOT_MAGIC_SIZE))) {
+					fastboot_fail("image is not a boot image");
+					return;
+				}
+
 				/* Reset multislot_partition attributes in case of flashing boot */
 				if (partition_multislot_is_supported())
 				{