x1000: Add multiboot support

Enables multiboot for the FiiO M3K, Shanling Q1, and Eros Q native port.
Note this requires an up-to-date Rockbox _and_ bootloader.

Usage instructions will (eventually) be found on the wiki:
https://www.rockbox.org/wiki/MultibootBootloader

Change-Id: Ia2da1ad6ef611e499d2fbafa93838387bc1023ba

firmware/target/mips/ingenic_x1000/crt0.S

@@ -21,6 +21,7 @@
 
 #include "config.h"
 #include "mips.h"
+#include "bootdata.h"
 
     .text
     .extern main
@@ -48,6 +49,11 @@ _header:
     .word   _loadaddress
     .ascii  "ENDH"          /* end of header structure */
 
+#ifndef BOOTLOADER
+    /* Multiboot support header; this is not part of the above header. */
+    put_boot_data_here
+#endif
+
 _realstart:
     /* Copy IRAM from BSS to low memory. */
     la      a0, _iramcopy