scripts: sercomm-kernel-header.py: improve compatibility

This improves compatibility with the elder stock firmwares of the
following devices, which have not yet been merged into OpenWrt:
 - Beeline SmartBox Pro
 - Beeline SmartBox Turbo+
 - WiFire S1500.NBN

Without this, OpenWrt factory image installation may fail.

Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>
Signed-off-by: Maximilian Weinmann <x1@disroot.org>

scripts/sercomm-kernel-header.py

@@ -48,12 +48,12 @@ def get_kernel_header(args):
 	struct.pack_into('<L', header, 0x2c, rootfs_size)
 	struct.pack_into('<L', header, 0x30, crc)
 
-	rootfs_end_offset = args.rootfs_offset + rootfs_size
-	struct.pack_into('<L', header, 0x4, rootfs_end_offset)
-
 	kernel_size = os.path.getsize(args.kernel_file)
 	struct.pack_into('<L', header, 0x14, kernel_size)
 
+	kernel_end_offset = args.kernel_offset + kernel_size
+	struct.pack_into('<L', header, 0x4, kernel_end_offset)
+
 	buf = open(args.kernel_file,'rb').read()
 	crc = binascii.crc32(buf) & 0xffffffff
 	struct.pack_into('<L', header, 0x18, crc)