Makefile: Force regeneration of env.txt

If the source .env file changes to one that is also older than the
generated env.txt file then the .env file is not regenerated. This
means when switching board configs we do not regenerate the env.

This can be tested with:

$ make j721e_evm_a72_defconfig
$ make # this may fail to complete but that is okay for this test
$ make am64x_evm_a53_defconfig
$ make
$ vim include/generated/env.txt

Note this is still the J721e env not the AM64 config as expected.

As ENV_FILE is set based on configuration, regenerate anytime
autoconf.h changes.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>