tools: docker: Build and install genimage

genimage [1] is a tool to create flash/disk images. This is required
by some targets, e.g.: sifive_unleashed, to generate sdcard or spi-nor
images for real hardware, as well as U-Boot CI testing.

[1] https://github.com/pengutronix/genimage



Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>