Have the system use env to find echo

[Krutonium]

On my (NixOS) system it was showing errors because the echo binary is not where it might normally be. Using /usr/bin/env should be universal and ensure that the echo binary can be found.

[NiKiZe]

This adds new dependency on /usr/bin/env in Makefile since it is not used anywhere before? How they are used in utils.

One could also argue that NixOS is the odd one here, but unless there is a good reason against this I think it should be valid to be merged. (after confirming to https://ipxe.org/admin, please update commit and PR, force push will be needed)

[Krutonium]

This adds new dependency on /usr/bin/env in Makefile since it is not used anywhere before? How they are used in utils.

One could also argue that NixOS is the odd one here, but unless there is a good reason against this I think it should be valid to be merged. (after confirming to https://ipxe.org/admin, please update commit and PR, force push will be needed)

I suppose it does, but I have yet to see a system that doesn't have /usr/bin/env, including on systems like mine where echo is located at /run/current-system/sw/bin/echo which is actually a symlink to /nix/store/x5f50z1kgdxmbjw65p84ifqybphc12c3-coreutils-full-9.0/bin/echo

Idea: Install /bin/echo as build dependency.

My websearch on NixOS /bin/echo yielded:

* [known problem](https://github.com/hmemcpy/milewski-ctfp-pdf/issues/208)

* [workaround](https://github.com/hmemcpy/milewski-ctfp-pdf/issues/209)

Perhaps using printf would indeed be cleaner, thoughts in general from others?