imx: Convert SERIAL_TAG support to ENV_VARS_UBOOT_RUNTIME_CONFIG

No iMX platforms have supported ATAG-based booting.  They have however
re-used the CONFIG_SERIAL_TAG option as a way to enable support of
reading the OTP fuses and setting the serial# environment variable in
some cases.  Change the warp7 support to use this symbol, use this for
updating the rest of the imx7 code, and update the imx8 conditionals.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP i.MX U-Boot Team <uboot-imx@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/mach-imx/imx8ulp/soc.c b/arch/arm/mach-imx/imx8ulp/soc.c
index 1c33acc..bba6323 100644
--- a/arch/arm/mach-imx/imx8ulp/soc.c
+++ b/arch/arm/mach-imx/imx8ulp/soc.c
@@ -405,7 +405,7 @@
 	return 0;
 }
 
-#ifdef CONFIG_SERIAL_TAG
+#ifdef CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG
 void get_board_serial(struct tag_serialnr *serialnr)
 {
 	u32 uid[4];