Force build configuration choice; update docs

It was getting to easy to choose the buildroot default. This commit also
adds info about caching downloads which is really important.
rpi: Enable hardware interfaces (gpio, i2c, spi)
erlinit: bump version to v0.2.0
rpi: add erlinit.config to force controlling tty

This is needed to select between seeing the prompt on HDMI vs. seeing
the prompt over the UART.
nerves-env: add erl_interface to the library list
syslinux patch just got accepted into buildroot