board/boundarydevices: promote buildroot-external-boundary project

It may be useful for users using Boundary Devices boards to find
more advanced defconfigs than 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>
(cherry picked from commit 75543322843f9ce72a4f39f1ee6239059c061e6c)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/boundarydevices/common/readme.txt

@@ -40,3 +40,9 @@ Where 'sdX' is the device node of the uSD partition.
 To upgrade u-boot, cancel autoboot and type:
 
 > run upgradeu
+
+See Boundary Devices's buildroot-external-boundary project
+for additional and advanced defconfigs using Qt5, gstreamer,
+NXP proprietary packages with demo applications:
+
+https://github.com/boundarydevices/buildroot-external-boundary