The net-wirless/broadcom-sta ebuild is affected by two issues currently.

• The bradcom website changed on Dec 4, 2016 and the URLs of the build are outdated. I've found the new URL of the driver, can't find the README. Will update when I find it.
  amd64: https://docs.broadcom.com/docs-and-downloads/docs/linux_sta/hybrid-v35_64-nodebug-pcoem-6_30_223_271.tar.gz
  x86: https://docs.broadcom.com/docs-and-downloads/docs/linux_sta/hybrid-v35-nodebug-pcoem-6_30_223_271.tar.gz

• The build needs patches for 4.7 and 4.8 linux kernels, gentoo has these currently in their ebuild: here