Add bootconfig to kernel cmdline when BOARD_BOOTCONFIG not empty

The 'bootconfig' kernel cmdline parameter needs to exist for the kernel
to search for bootconfig.
If BOARD_BOOTCONFIG contains anything, then add the parameter to the
kernel cmdline.

Test: Boot cuttlefish and verify /proc/cmdline has 'bootconfig' after
removing it from cuttlefish BoardConfig.mk
Bug: 173815685

Change-Id: I112a2a8e02ba7265c5547d9244298e07f26985ba