meta-google: create new partition for hoth-secondary

Tested:
After build one platform,
cr51-image-layout.json:
```
{
        "length": 55508992,
	"name": "rofs",
	"offset": 7340032,
	"region_type": [
		"STATIC",
		"WRITE_PROTECTED"
	]
},
,{
        "length": 1048576,
        "name": "hoth_secondary",
	"offset": 62849024,
        "region_type": []
}
```

Change-Id: Id7869f96ff65625547ff83640e300b2202c4aab8
Signed-off-by: Claire Liao <claireliao@google.com>
diff --git a/meta-google/classes/image_types_hoth.bbclass b/meta-google/classes/image_types_hoth.bbclass
index 5ad64f9..e0bce25 100644
--- a/meta-google/classes/image_types_hoth.bbclass
+++ b/meta-google/classes/image_types_hoth.bbclass
@@ -3,6 +3,7 @@
 FLASH_IMAGE_DESC_OFFSET:hoth = "${@960 if FLASH_SIZE == '65536' else 7232}"
 FLASH_HOTH_UPDATE_OFFSET:hoth = "${@1024 if FLASH_SIZE == '65536' else 31744}"
 FLASH_HOTH_MAILBOX_OFFSET:hoth = "${@65472 if FLASH_SIZE == '65536' else 7168}"
+FLASH_HOTH_SECONDARY_OFFSET:hoth = "${@61376 if FLASH_SIZE == '65536' else 7296}"
 
 # 64 bit kernels are larger, so they require a different layout
 FLASH_IMAGE_DESC_OFFSET:hoth:aarch64 = "${@61312 if FLASH_SIZE == '65536' else 7232}"
@@ -112,10 +113,14 @@
             convertPart(
                     'rofs',
                     d.getVar('FLASH_ROFS_OFFSET'),
-                    d.getVar('FLASH_RWFS_OFFSET'),
+                    d.getVar('FLASH_HOTH_SECONDARY_OFFSET'),
                     static=True,
                     wp=True),
             convertPart(
+                    'hoth_secondary',
+                    d.getVar('FLASH_HOTH_SECONDARY_OFFSET'),
+                    d.getVar('FLASH_RWFS_OFFSET')),
+            convertPart(
                     'rwfs',
                     d.getVar('FLASH_RWFS_OFFSET'),
                     d.getVar('FLASH_HOTH_MAILBOX_OFFSET'),