regulators: Complete compare_vpd action

Complete implementation of the compare_vpd action in the JSON
configuration file.  For more information about this action see
https://github.com/openbmc/phosphor-power/blob/master/phosphor-regulators/docs/config_file/compare_vpd.md

Implement the execute() method of the CompareVPDAction class.  Obtain
the actual VPD keyword value from the VPD service and compare it with
the expected value.

Also improve doxygen for the execute() method of the
ComparePresenceAction class.

Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
Change-Id: Ieb93806245babe6782fef95209bed8eed5b32578
diff --git a/phosphor-regulators/src/meson.build b/phosphor-regulators/src/meson.build
index d7fc414..9993591 100644
--- a/phosphor-regulators/src/meson.build
+++ b/phosphor-regulators/src/meson.build
@@ -24,6 +24,7 @@
     'vpd.cpp',
 
     'actions/compare_presence_action.cpp',
+    'actions/compare_vpd_action.cpp',
     'actions/if_action.cpp',
     'actions/i2c_compare_bit_action.cpp',
     'actions/i2c_compare_byte_action.cpp',