board/boundarydevices: promote buildroot-external-boundary project

It may be useful for users using Boundary Devices boards to find
more advanced defconfigs that the one provided by Buildroot.

See:
https://github.com/boundarydevices/buildroot-external-boundary#configurations-details



Update the readme.txt to add the link to the br2_external maintained
by Boundary Devices.

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>