arm: imx: imx9: soc: Fix env location when booting from USB

arm: imx: imx9: soc: Fix env location when booting from USB

On i.MX9 platforms, when booting from USB, the U-Boot environment is
always assumed to be in RAM. However, this causes the boot to hang when
`CONFIG_ENV_IS_NOWHERE` is not enabled. The boot also hangs even if the
environment is present in another storage media (for example, eMMC). Fix
the issue by correctly handling the U-Boot environment's location when
booting from USB. Also for i.MX95, set the environment location based on
the ENV config and not solely based on the boot device type.

Suggested-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: João Paulo Gonçalves <joao.goncalves@toradex.com>