* Remove CONFIG_CLOCKSOURCE_EFI configuration
It seems to cause messages like this on some machines:
EFI Event timer too slow freq = 100 Hz
The Barebox efi_defconfig configurationd doesn't enable it either.
Disable it by default as well.
* Enable CONFIG_CMD_ECHO_E to fix menutree
It seems that menutree needs CONFIG_CMD_ECHO_E to properly display the
boot menu.
Also enable other useful commands such as edit or reset.
* Bump Barebox to 2021.05.0
* Rename Intel NUC machine to Generic x86-64
The Intel NUC machine has evolved and supports various x86-64 machines
today. Rename the board.
Note that this does not address the migration issue. This will be
handled separately.
* Update Scripts/Documentation
