[linux] Use dummy SAN device

Allow for easier testing of SAN code by using the dummy SAN device by
default.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/defaults/linux.h

 #define UMALLOC_LINUX
 #define NAP_LINUX
 #define SMBIOS_LINUX
-#define SANBOOT_NULL
+#define SANBOOT_DUMMY
 #define ENTROPY_LINUX
 #define TIME_LINUX
 #define REBOOT_NULL
 
 #define IMAGE_SCRIPT
 
+#define SANBOOT_PROTO_ISCSI
+#define SANBOOT_PROTO_AOE
+#define SANBOOT_PROTO_IB_SRP
+#define SANBOOT_PROTO_FCP
+#define SANBOOT_PROTO_HTTP
+
 #endif /* CONFIG_DEFAULTS_LINUX_H */