toolchain helpers: introduce function relpath_prefix

The helper function copy_toolchain_sysroot has some logic to transform a
path into a number of '../' components based on the depth of that path.

As this same logic will be needed in another place in a subsequent patch,
extract it into a separate helper relpath_prefix.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>