env: Migrate CONFIG_ENV_IS_IN_FAT options to Kconfig

We rename the various FAT_ENV_xxx options to CONFIG_ENV_FAT_xxx so that
they can be modified via Kconfig.  Migrate all existing users to the new
values.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/axs103_defconfig b/configs/axs103_defconfig
index ff0bf19..68301ef 100644
--- a/configs/axs103_defconfig
+++ b/configs/axs103_defconfig
@@ -4,6 +4,8 @@
 CONFIG_SYS_CLK_FREQ=100000000
 CONFIG_DEFAULT_DEVICE_TREE="axs103"
 CONFIG_ENV_IS_IN_FAT=y
+CONFIG_ENV_FAT_INTERFACE="mmc"
+CONFIG_ENV_FAT_DEVICE_AND_PART="0:1"
 CONFIG_BOOTDELAY=3
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SYS_PROMPT="AXS# "