configs/stm32mp157_dk: new defconfig

configs/stm32mp157_dk: new defconfig

This commit adds a minimal defconfig to build a Buildroot system for
the STM32MP157 Discovery Kit, which is the low-cost evaluation
platform for the new STM32MP157 system-on-chip from ST. This
system-on-chip features a single or dual Cortex-A7, a single
Cortex-M4, and a wide variety of peripherals.

This commit includes a custom linux configuration file, because there
is no specific defconfig for this platform in the kernel, only
multi_v7_defconfig supports it, which is really a massive
defconfig. That's why a custom linux configuration file is added.

A small U-Boot config fragment allows to disable the watchdog, so that
userspace by default doesn't need to have a watchdog daemon running.

The vendor U-Boot and Linux trees are used for the moment, but the
platform support is being upstreamed, so switching to upstream
versions will be possible in the relatively near future.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>