config: Use booti instead of bootz on 64-bit ARM

The bootz command doesn't work with Linux kernel images on 64-bit ARM.
The replacement command with the same interface and functionality is
booti.

Cc: Dennis Gilmore <dennis@ausil.us>
Cc: Tom Rini <trini@konsulko.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>

include/config_distro_defaults.h

@@ -31,7 +31,11 @@
 
 #define CONFIG_OF_LIBFDT
 
+#ifdef CONFIG_ARM64
+#define CONFIG_CMD_BOOTI
+#else
 #define CONFIG_CMD_BOOTZ
+#endif
 #define CONFIG_CMD_DHCP
 #define CONFIG_CMD_ELF
 #define CONFIG_CMD_EXT2