build: Specify a list of directories to create

Some components will need some empty directories in the tree which
their build system won't create.

An example is dbus which requires /etc/dbus-1/{system,session}.d
directories.

With the makedirs entry in the component's definition we will be able
to create such directories.
Directories will be created right after the build results directory has
been processed so names must start with the target in their name,
for example /runtime/etc/dbus-1/session.d and
/runtime/etc/dbus-1/system.d.

Closes #34