PEL: Add function to get PEL size

Add a size() function to the PEL class that adds up the size fields
in the header of every PEL section.

This required a fix to some testcases where the size field was wrong in
a header.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I0d70deae116cd3835f2c0ab34e13811da471fb14
diff --git a/test/openpower-pels/pel_utils.cpp b/test/openpower-pels/pel_utils.cpp
index 18095a1..4560b2f 100644
--- a/test/openpower-pels/pel_utils.cpp
+++ b/test/openpower-pels/pel_utils.cpp
@@ -66,7 +66,7 @@
 const std::vector<uint8_t> srcSectionNoCallouts{
 
     // Header
-    'P', 'S', 0x00, 0x80, 0x01, 0x01, 0x02, 0x02,
+    'P', 'S', 0x00, 0x50, 0x01, 0x01, 0x02, 0x02,
 
     0x02, 0x00, 0x00, // version, flags, reserved
     0x09, 0x00, 0x00, // hex word count, reserved2B