fastboot: add support for 'getvar platform'

Our R-Car3 Android userspace relies on the output of 'fastboot
getvar platform' and U-Boot currently is not able to handle it:

host $> fastboot getvar platform
getvar:platform FAILED (remote: Variable not implemented)
finished. total time: 0.001s

We either have the option of adding 'fastboot.platform' variable
to the default/saved environment as a workaround or add proper
'fastboot getvar platform' support in U-Boot via this patch.
In the latter case, other platforms can benefit from it too.

Note that R-Car3 already exports 'platform' environment variable via
v2019.01 commit 00e4b57e ("ARM: rmobile: Set environment variable
containing CPU type").

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>