init: handle more bootconfig parameters

As parameters are moved from kernel cmdline to bootconfig,
first_stage_init needs to be updated to handle the new
location.
/proc/bootconfig should be checked first, if not present, then check
/proc/cmdline.

Test: launch_cvd
Test: launch_cvd with 4.19 kernel artifacts that do not support
bootconfig
Test: Both of the above configurations with --num_instances 0 or 4
Test: Both configurations with androidboot.boot_devices or
androidboot.boot_device set
Bug: 173815685

Change-Id: I03743f922351d58375e8b9a903899b8bc54bd71e