need to support PIP from multiple fcs builds of the same version of the JRE

When we build patches for Patch-in-Place (PIP), we currently only support building 1 patch per version of the JRE. We need to support patching multiple fcs builds of the same JRE version.

Let's say 6u18-b10 (fcs) goes out to the public and 9 million get this JRE. Then we find a major flaw somewhere in the JRE and management wants to spin a new 6u18 fcs version 6u18-b11 (fcs). 9 million people will still have b10, and the rest of the population will have b11. This is currently fine, until we have to build 6u19 and later releases. 6u19 can only currently patch one version of the 6u18 release. If we build 6u19 patching the 6u18-b11 version, then auto-updating the 6u18-b10 fcs users to 6u19 will require a full 9MB download (rather than a usual 1MB patch). And vice-versa, if we build 6u19 patching the 6u18-b10 version, then auto-updating the 6u18-b11 fcs users to 6u19 will require a full 9MB download. This is extremely expensive not only for paying our CDNs, but also for maintaining suitable bandwidth on the server side.
This RFE does not include patching from 6u18-b10 (fcs) to 6u18-b11 (fcs).