installboot(8) to write u-boot images to SPI NOR and other types of
raw flash devices: input-block-size, input-pad-size, output-size, and
output-block-size.
Add the ability to create aliases for install objects, useful for when
e.g. sdmmc and emmc share the same steps.
Tested on an A20-OLinuXino-LIME2-e16Gs16M by bouyer@.
XXX pullup netbsd-9