configs/hifive_unleashed: new defconfig

configs/hifive_unleashed: new defconfig

This patch adds support for the HiFive Unleashed board. This includes
building the firmware, kernel and rootFS for the HiFive Unleashed.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
[Thomas:
 - fix the directory name hifive_unleased -> hifive-unleashed
 - drop from readme.txt the instructions about manually flashing each
   partition, since we have a full SD card image
 - drop the custom post-image.sh script]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>