tools: blob_handler: add some const ness
tools: blob_handler: add some const ness

Many pointers cast that should be const by use-case were missing this.

Change-Id: Ib75a210d731d731ef07f59d4132f70e5cfd324bf
Signed-off-by: Patrick Venture <36c53fa43f47f3fd67cb2fc38031827244d18a1f@google.com>

openbmc/phosphor-ipmi-flash