layer.conf: Fix u-boot-imx overrides for i.MX 8

u-boot-imx builds are broken for i.MX 8:

```
install: cannot stat '.../tmp/work/imx8qm_mek-poky-linux/u-boot-imx/1_2020.04-r0/git/imx8qm_mek_defconfig/spl/u-boot-spl.bin': No such file or directory
```

This is because the overrides added recently for UBOOT_MAKE_TARGET and
UBOOT_SUFFIX are incorrect for i.MX 8. Limit the overrides to i.MX 6
and 7.

Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>

Author: thochstein

meta-bsp/conf/layer.conf

@@ -45,8 +45,10 @@ IMX_DEFAULT_BSP = "nxp"
 # override it here.
 IMX_DEFAULT_BOOTLOADER_imx = "u-boot-imx"
 
-UBOOT_MAKE_TARGET_pn-u-boot-imx ?= "u-boot.${UBOOT_SUFFIX}"
-UBOOT_SUFFIX_pn-u-boot-imx ?= "imx"
+UBOOT_MAKE_TARGET_pn-u-boot-imx_mx6 = "u-boot.imx"
+UBOOT_MAKE_TARGET_pn-u-boot-imx_mx7 = "u-boot.imx"
+UBOOT_SUFFIX_pn-u-boot-imx_mx6 = "imx"
+UBOOT_SUFFIX_pn-u-boot-imx_mx7 = "imx"
 
 # Avoid multiple runtime providers for u-boot-default-env
 PREFERRED_RPROVIDER_u-boot-default-env ??= "${IMX_DEFAULT_BOOTLOADER}"