encoding: Extending Block and EncodingBuffer interfaces

Now it is possible to use constructors to create Block from
EncodingBuffer and EncodingBuffer from Block.

Block->EncodingBuffer conversion is potentially dangerous and should be
used only in exceptional cases, such as Data packet encoding: to encode
the signed part first and then extend signed part with signature and
other related fields.

Change-Id: I5a13bf0c196ecd0d45dfa14c4cb6f4a9f612420c
build: Extending wscript to enable coverage information with --with-coverage option

When coverage option is enabled, all necessary flags are exported via
pkgconfig configuration.

Change-Id: Ieac8c17b2b81568fddbfd61d99bb79a5e2b99f13
refs: #1215