move files around to create a common convenience library

Problem: plan is to add another blob handler into ipmi-flash
(ipmi-flash-version). This new handler will re-use much of the
i

move files around to create a common convenience library

Problem: plan is to add another blob handler into ipmi-flash
(ipmi-flash-version). This new handler will re-use much of the
ipmi-flash (firmware-handler) code. The common code should be presented
as a convenience library to reduce code duplication.

Solution: move anticipated firmware-handler specific code into the
subdirectory bmc/firmware-handler and leave common code in bmc/.

The end goal is to have version-handler re-use as
much code as possible.

Tested:
rebuilt everything and ran unit tests.

Signed-off-by: Jason Ling <jasonling@google.com>
Change-Id: I2128da629b0ddf27b89f1faee358d1941f1dff38

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