epics: Add parsing of epicsdb_* fields to generate custom EPICS db fields

This commit lets the user define the fields in the .wb that are going to
forwarded to the EPICS database.

if we have in the *.wb

~~~~~~~~~~~~{.wb}
      field {
         name = "Amplitude";
         ...
         epicsdb_egu  = "V";
         epicsdb_hopr = "1.5";
      };
~~~~~~~~~~~~

We will obtain in the *.db

~~~~~~~~~~~~{.wb}
record(ao, "$(user):AdcAmpSet")
{
	...
	field(DESC ,"Amplitude")
	field(EGU  ,"V")
	field(HOPR ,"1.5")
}
~~~~~~~~~~~~