Convert CONFIG_ENV_IS_IN_FAT to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_FAT

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/picosam9g45_defconfig b/configs/picosam9g45_defconfig
index aa61e4b..326158d 100644
--- a/configs/picosam9g45_defconfig
+++ b/configs/picosam9g45_defconfig
@@ -10,6 +10,7 @@
 CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_SPL_FAT_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9M10G45,SYS_USE_MMC"
+CONFIG_ENV_IS_IN_FAT=y
 CONFIG_BOOTDELAY=3
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y