tools/genboardscfg.py: Make 'Supported' as known status

As per MAINTAINERS[1] file description, 'Supported' is
a valid status for a board. But buildman thinks 'Maintained'
is the only valid state and complains about boards with 'Supported'
status. Update buildman to accept 'Supported' as valid state.

[1] http://git.denx.de/?p=u-boot.git;a=blob;f=MAINTAINERS;h=0962b47bf9057b22e93624e070c0204b893790dc;hb=HEAD#l10



Reported-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>