feat: add Sky Xboot image signature

Found in wifi router WNXL11BWL.

Author: cpatulea

mime/firmware_containers

@@ -972,3 +972,11 @@
 0       search/0x1000   VER3Puma7_UImage\x00\x00\x00\x00\x00\x00\x00\x00        Intel (MaxLinear) UImage v3,
 !:mime  firmware/maxlinear-uimage-v3
 >&12    ubelong         x                                                       size: %d
+
+# Sky Xboot
+0       search/0x1000   \x00\x00\x00\x03----MANIFEST----      Sky Xboot manifest v3,
+>&0     pstring/L       x                                     package type: %s,
+>>&0    pstring/L       x                                     code version: %s,
+# secure version number
+>>>&0   ubelong         x
+>>>&4   pstring/L       x                                     model name: %s