env: Implement lower priority arch_env_get_location()

Currently there is only one way to override desired environment location,
by implementing env_get_location(). This is increasingly being conflated
both on board level and architecture level, which leads to a problem on
boards where this function is already implemented on architecture level,
since those boards have no way to override this environment location on
board level anymore.

Implement arch_env_get_location() function which is architecture specific
and should only ever be implemented in architecture code. This function
has lower priority than env_get_location(), which should only ever be
implemented in board code, and which overrides the arch_env_get_location()
architecture environment selection.

This way, architecture can define its default environment chooser, while
board can now override it as needed at all times.

There is no functional change, since env_get_location() simply returns
arch_env_get_location(), and arch_env_get_location() implements the
current env_get_location() default content.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Adam Ford <aford173@gmail.com>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Marek Behún <marek.behun@nic.cz>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Tommaso Merciai <tomm.merciai@gmail.com>
Cc: Vladimir Oltean <vladimir.oltean@nxp.com>