qemu: update u-boot config fragment for v2019.01 to v2020.04

Update U-Boot config fragment to explicitly state no flash or MTD device
is used by U-Boot and the U-Boot environment is not stored outside
U-Boot image. This change is required since U-Boot v2019.01. It
successfully boots Qemu/armv7-A with older and newer U-Boot releases up
to v2020.04, and at least also up to latest U-boot RC tag v2020.07-rc3.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Joakim Bech <joakim.bech@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>