checkseapp: generalize input validation

Input validation was hard-coded into a validation routine
that would check against type and key names in a scattered,
order dependent conditional code block.

This makes it harder than it should be to add new key value
pairs and types into checkseapp.

To correct this, we add a validation callback into the
static mapping. If the validation callback is set, the
existing validation routine will call this for input
validation. On failure, a validation specific error message
is returned to be displayed.

Change-Id: I92cf1cdf4ddbcfae19168b621f47169a3cf551ac
Signed-off-by: William Roberts <william.c.roberts@intel.com>