doc: Fix terminology in Kconfig files for 'platform'

Completing the terminology change started with change 4008
by updating the Kconfig files processed to produce the
online documentation, plus header files processed by
doxygen.  References to 'platform' are change to 'board'

Change-Id: Id0ed3dc1439a0ea0a4bd19d4904889cf79bec33e
Jira: ZEP-534
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>