rockchip: puma-rk3399: enforce ENV_IS_NOWHERE with Kconfig select

Instead of letting the compiler error out if CONFIG_ENV_IS_NOWHERE is
not selected by the user, let's just enforce it when the user builds for
Puma RK3399 so that no check needs to be performed by the compiler and
the configuration is always valid.

Suggested-by: Tom Rini <trini@konsulko.com>
Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/board/theobroma-systems/puma_rk3399/Kconfig b/board/theobroma-systems/puma_rk3399/Kconfig
index 15af555..cc745f5 100644
--- a/board/theobroma-systems/puma_rk3399/Kconfig
+++ b/board/theobroma-systems/puma_rk3399/Kconfig
@@ -11,6 +11,7 @@
 
 config BOARD_SPECIFIC_OPTIONS # dummy
 	def_bool y
+	select ENV_IS_NOWHERE
 
 config ENV_SIZE
 	default 0x4000