rockchip: rk3399-puma: reduce env size to 8kiB

This commit changes the size of the enviroment (for the RK3399-Q7) to
8kiB for all possible locations of the environment (i.e. even when the
environment is saved to SD card).

With the default of 32kiB, the environment overwrites the SPL
stage which lives at 16kiB.
Signed-off-by: NJakob Unterwurzacher <jakob.unterwurzacher@theobroma-systems.com>
Acked-by: NPhilipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: NPhilipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: NPhilipp Tomsich <philipp.tomsich@theobroma-systems.com>
[Reworked commit-message:]
Signed-off-by: NPhilipp Tomsich <philipp.tomsich@theobroma-systems.com>

board/theobroma-systems/puma_rk3399/Kconfig

@@ -13,7 +13,7 @@ config BOARD_SPECIFIC_OPTIONS # dummy
 	def_bool y
 
 config ENV_SIZE
-	default 0x2000 if ENV_IS_IN_SPI_FLASH
+	default 0x2000
 
 config ENV_OFFSET
 	default 0x3c000 if ENV_IS_IN_SPI_FLASH