buildroot

configs/atmel: bump to linux4sam_5.8

Bump at91sam9x5ek, atmel_sama5d27_som1_ek, atmel_sama5d2_xplained,
atmel_sama5d3_xplained, and atmel_sama5d4_xplained all variants to
linux4sam_5.8. The 3 foundation components have their tags changed
(AT91Bootstrap, U-Boot, Linux kernel).

at91bootstrap 3.8.10 is required to support gcc7.

Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>