spl: mmc: remove #ifdef CONFIG_SPL_OS_BOOT check

Implement default versions of falcon mode functions to make the
CONFIG_SPL_OS_BOOT check in spl_mmc_load_image() unnecessary, thus reducing
its #ifdef complexity.

No functional changes.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Paul Kocialkowski <contact@paulk.fr>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Guillaume GARDET <guillaume.gardet@free.fr>
Cc: Suriyan Ramasami <suriyan.r@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/common/spl/spl_ext.c b/common/spl/spl_ext.c
index 9d37fd3..a42fbd0 100644
--- a/common/spl/spl_ext.c
+++ b/common/spl/spl_ext.c
@@ -6,6 +6,7 @@
 #include <spl.h>
 #include <asm/u-boot.h>
 #include <ext4fs.h>
+#include <errno.h>
 #include <image.h>
 
 #ifdef CONFIG_SPL_EXT_SUPPORT
@@ -135,5 +136,10 @@
 	return spl_load_image_ext(block_dev, partition,
 			CONFIG_SPL_FS_LOAD_KERNEL_NAME);
 }
+#else
+int spl_load_image_ext_os(block_dev_desc_t *block_dev, int partition)
+{
+	return -ENOSYS;
+}
 #endif
 #endif