system: introduce new attribute board_name

system: introduce new attribute board_name

The system board call returns the verbose model string instead of the
board name. Unfortunately we have not yet estabkushed clear convention
regarding device-tree 'compatible' or 'model' property and sysupgrade
image name (same accounts for scraping /proc/cpuinfo on legacy
targets). This is odd as the idea was to return information needed to
identify the right sysupgrade image. On most targets we got a large
shell-script which translates either /proc/cpuinfo or the 'model'
property back into the board name used here.
Hence introduce a new attribute board_name to return either
/tmp/sysinfo/board_name or /proc/device-tree/compatible.

This combines commit 79bbe6d (system: return legacy board name) and
commit 453116e (system: introduce new attribute board_name) from the
master branch into a single commit.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>