fix(wapm) Fix the wax script generation

[Hywan]

Description

On macOS, echo always enables interpretation of backslach sequences
(like \n). On Linux, the -e flag must be used. Even if this -e
flag is absent from the macOS man's page for echo(1), it is accepted
and ignored, thus we can safely use it in our Makefile.

To test this patch:

$ # before
$ make package-wapm
$ cat package/bin/wax
#!/bin/bash\nwapm execute "$@"
$
$ # after
$ make package-wapm
$ cat package/bin/wax
#!/bin/bash
wapm execute "$@"

This patch fixes #2425.

Review

• Add a short description of the change to the CHANGELOG.md file

[syrusakbary]

Awesome! Merging manually