hw/lm32/Kconfig: Introduce CONFIG_LM32_EVR for lm32-evr/uclinux boards

We want to be able to use the 'LM32' config for architecture
specific features. Introduce CONFIG_LM32_EVR to select the
lm32-evr / lm32-uclinux boards.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210221225626.2589247-2-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>