wic: Introduce empty plugin to create unformatted empty partitions

wic: Introduce empty plugin to create unformatted empty partitions

The empty wic plugin is used to create unformatted empty partitions for wic
images.

To use it you must pass "empty" as argument for the "--source" parameter in
the wks file. For example:
    part foo --source empty --ondisk sda --size="1024" --align 1024

Also adds a selftest for this plugin where the 'Fstype' column from 'wic
ls' should be empty for the second partition as listed in
test_empty_plugin.wks.

Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>