build: pkg-config required to build kconfig-frontends

Add it to the list of dependencies for building kconfig-frontends.

Change-Id: Ifdce2364f18557ed39888386c2c3032b2b6258ee
doc: add chip doc pages

Add documentation for each supported chip series.  In the docs, explain
what peripherals are supported, what boards are officially supported,
how to flash or boot the chip, and where to find stdout and stderr.

Change-Id: I4a520cf92a3a6a75985a4a89aff4fba7dea44225