build: be forgiving to missing newlines in manifest

Running "make image=mgmt,iperf" failed because the last line in
apps/iperf/usr.manifest did not have a newline, and was copied
without a newline, which caused it to be stuck together to the first
line of mgmt's manifest.

Fix this by explicitly adding a newline character to each line we add to
the generated manifest file - whether or not the original manifest had one.

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>