PEL: Add class to wrap AdditionalData

The AdditionalData property on the xyz.openbmc_project.Logging.Entry
interface is a vector of strings of the form:  "KEY=VALUE".  The
PEL processing code will be interested in those keys and values, and
this class adds a way to get at those values based on a key without
having to do string parsing each time.  It returns an
std::optional<std::string> value, and if the key isn't found, then the
std::optional value will be empty.

For Example:
    AdditionalData ad{additionalDataPropertyValue};

    // Get the value for the FOO key
    std::optional<std::string> val = ad.getValue("FOO");

    if (val)
        std::cout << (*val).size();

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I6ba458840278784b1cc6a0ed88a7fece8794df7d
diff --git a/test/openpower-pels/Makefile.include b/test/openpower-pels/Makefile.include
new file mode 100644
index 0000000..98a0acd
--- /dev/null
+++ b/test/openpower-pels/Makefile.include
@@ -0,0 +1,10 @@
+TESTS += $(check_PROGRAMS)
+
+check_PROGRAMS += \
+	additional_data_test
+
+additional_data_test_SOURCES = %reldir%/additional_data_test.cpp
+additional_data_test_CPPFLAGS = $(test_cppflags)
+additional_data_test_CXXFLAGS = $(test_cxxflags)
+additional_data_test_LDADD = $(test_ldadd)
+additional_data_test_LDFLAGS = $(test_ldflags)