buildroot

configs: atmel: xplained boards: bump to linux4sam_5.5

Bump at91bootstrap to v3.8.7, U-Boot to linux4sam_5.5, Linux to linux4sam_5.5
(based on v4.4.x).

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>