configs: meson64: permit redefining ENV_SIZE

Permit redefining ENV_SIZE for board specific configs.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
diff --git a/include/configs/meson64.h b/include/configs/meson64.h
index 78b1615..f8d3eee 100644
--- a/include/configs/meson64.h
+++ b/include/configs/meson64.h
@@ -37,7 +37,9 @@
 
 #define CONFIG_CPU_ARMV8
 #define CONFIG_REMAKE_ELF
+#ifndef CONFIG_ENV_SIZE
 #define CONFIG_ENV_SIZE			0x2000
+#endif
 #define CONFIG_SYS_MAXARGS		32
 #define CONFIG_SYS_MALLOC_LEN		(32 << 20)
 #define CONFIG_SYS_CBSIZE		1024