fastboot: allow retrieving fastboot variables from env

fastboot: allow retrieving fastboot variables from env

Some boards need to expose device specific variable through fastboot
(to adpat the flashing script depending on hardware revision for
example).

Provide a way to expose custom fastboot variables. Note that all
variables meant to be exposed through fastboot should be be prefixed
with 'fastboot.', the variable should not exceed 32 bytes (including
the prefix and the trailing '\0') and the variable content should
fit in the response buffer (60 bytes excluding the 'OKAY' prefix and
the trailing '\0').

Signed-off-by: Rob Herring <rob.herring@linaro.org>
[Boris Brezillon: add a commit message]
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Steve Rae <srae@broadcom.com>