scripts/lib/wic/engine: Fix cp's target path for ext* filesystems

scripts/lib/wic/engine: Fix cp's target path for ext* filesystems

Python subprocess' shell=True defaults to /bin/sh[1][2], which
often refers to a POSIX-compliant shell. As the -e flag is
not defined in the POSIX standard[3], some shells may
interpret "-e" as the first argument to the echo command,
causing the rest of the command line to fail.

In this particular case, "echo -e 'cd {}'" is interpreted
as "-e cd {}", which causes the first line of the command
to fail, and causing cp to always place the source file
in the filesystem's root.

Replacing "echo -e" for a printf command makes this
more portable.

This issue only affects "wic cp" for ext* filesystems.

[1] https://docs.python.org/2/library/subprocess.html
[2] https://docs.python.org/3/library/subprocess.html
[3] http://pubs.opengroup.org/onlinepubs/9699919799

(From OE-Core rev: 9a5b703d4e60c3ccdf80b5289b8e6fc91133fcde)

Signed-off-by: Leonardo Augusto <leobsd@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>