fel: add progress bar support for uboot command

[gworkman]

The sunxi-fel uboot (and spl) commands lacked the ability to show a progress bar when uploading the payload, unlike the write command.

Support the -p / --progress flag by passing a progress callback through aw_fel_process_spl_and_uboot to aw_fel_write_uboot_image, which then sets up the progress indicator before calling aw_write_buffer.

Result:

$ sunxi-fel -p uboot path/to/u-boot-loader.bin
100% [================================================]   535 kB,  450.4 kB/s

[apritzel]

Is there any particular need for that? I see that's somewhat inconsistent with the other transfer commands, but as your screenshot above shows, U-Boot typically uploads within one second, so I am not sure we really need a progress bar? The moment you look at it, it's typically finished already.