Add kern.features flag for MMCCAM

[kibab]

Summary

kern.features.mmcam will be present and equal to 1 if the kernel has been compiled with option MMCCAM.
This will help sdio-related userland tools to fail-fast if running on the kernel without MMCCAM enabled.

Test Plan

• Compile GENERIC-MMCCAM of any other kernel with 'options MMCCAM';
• Upon booting, type sysctl kern.features.mmccam, the output should be 1.
• If running on non-MMCCAM kernel, there will be an error.