Allow providing default environment from file

Modifying the default environment via CONFIG_EXTRA_ENV_SETTINGS is
somewhat inflexible, partly because the cpp language does not allow
appending to an existing macro. This prevents reuse of "environment
fragments" for different boards, which in turn makes maintaining that
environment consistently tedious and error-prone.

This implements a Kconfig option for allowing one to define the entire
default environment in an external file, which can then, for example, be
generated programmatically as part of a Yocto recipe, or simply be kept
in version control separately from the U-boot repository.

Tested-by: Sean Nyekjaer <sean.nyekjaer@prevas.dk>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Lukasz Majewski <lukma@denx.de>