x1000: add simple binary header to declare the load address

The header format is basic, a variable length list of values
tagged by 4-byte strings. The main application to to allow a
loader to determine the load address of the binary at runtime
instead of using a hardcoded address. But the header contents
aren't fixed and it could be extended for other purposes too.

Change-Id: If51d85b4c2eb53c919a91b54985b157e8e5c20ef

firmware/target/mips/ingenic_x1000/crt0.S

@@ -25,6 +25,7 @@
     .text
     .extern main
     .extern system_early_init
+    .extern _loadaddress
     .global _start
 
     .set push
@@ -35,6 +36,19 @@
     .section .init.text
 
 _start:
+    b       _realstart
+    nop
+
+    /* Header entries are 4-byte string labels (not null terminated!) followed
+     * by 4-byte values. Header should begin in the first 128 bytes and should
+     * be no more than 256 bytes in length. */
+_header:
+    .ascii  "BEGINHDR"      /* beginning of header */
+    .ascii  "LOAD"
+    .word   _loadaddress
+    .ascii  "ENDH"          /* end of header structure */
+
+_realstart:
     /* Cache init */
     li      v0, 0x80000000
     ori     v1, v0, 0x4000