Convert CONFIG_ENV_IS_IN_FLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_FLASH

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/pm9261_defconfig b/configs/pm9261_defconfig
index ba079de..4c35f41 100644
--- a/configs/pm9261_defconfig
+++ b/configs/pm9261_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_AT91=y
 CONFIG_TARGET_PM9261=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9261"
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=3
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y