apphandler: Implement Get/Set System Info Parameter

Implement Get System Info Parameter using the parameter storage code to
back the string-type parameters. Supports up to 255 chunks (known as
"sets" in the spec) for those parameters. Currently, iterated reads by
chunk of a string parameter will repeatedly invoke that parameter's
callback, which can result in chunks being incoherent with each other if
the string changes between invocations. This is noted in a TODO comment.

Stub out Set System Info Parameter. Full implementation for that is
pending support for read-only flags in the parameter storage code.

Change-Id: If0a9d807725ccf1f1f62e931010024841575469c
Signed-off-by: Xo Wang <xow@google.com>
Signed-off-by: Patrick Venture <venture@google.com>
3 files changed