graphs: rename user-facing variables

Rename the GRAPH_OUT and GRAPH_ALT variables according to our
recently-agreed naming scheme for user-facing variables:
  - GRAPH_OUT -> BR2_GRAPH_OUT
  - GRAPH_ALT -> BR2_GRAPH_ALT

The documentation part of the rename is handled by Thomas as
part of his manual fixing spree. ;-)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>