kernel: Use correct variable name for vendor_kernel_boot modules

* This was missed in bdfc913935c2a8c38ed999a4c84bb96492e0e5fe
* Aligns the variable name for vendor_kernel_boot modules with AOSP

Change-Id: I8d59f7b5d62e29a0484ca36fadcb74acab4f1859
1 file changed