Accton platforms: Add context of installing u-boot in INSTALL

The platforms are including:
- as4600_54t
- as5600_52x
- as5610_52x
- as5710_54x
- as6700_32x
- as6701_32x
- as6710_32x
- as7700_32x

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>