gryphon-led-module: Build against bcmkernel on Broadcom platforms

Due to the complexity of the Broadcom build system which amongst others puts dynamically generated symlinks into the kernel dir, we have a fake kernel "bcmlinux" to please the OpenWrt build system. This kernel is not used for anything but building the OpenWrt toolchain. To compile against the right kernel, we need to compile the kernel module against the kernel inside bcmkernel which is the one compiled by the BCM SDK.