[objcopy] Refactor CommonConfig to add posibility to specify added/updated...

[objcopy] Refactor CommonConfig to add posibility to specify added/updated sections as MemoryBuffer.

Current objcopy implementation has a possibility to add or update sections.
The incoming section is specified as a pair: section name and name of the file
containing section data. The interface does not allow to specify incoming
section as a memory buffer. This patch adds possibility to specify incoming
section as a memory buffer.

Differential Revision: https://reviews.llvm.org/D120486