Set env vars from values on the efi loader command line.

Examine each cmdline arg and if it contains an '=' convert it to ascii and
pass it to putenv(). This allows var=value settings to come in on the
command line.

This will allow overriding dhcp server-provided data in loader(8), as
discussed in PR 202098

PR: 202098
Differential Revision: https://reviews.freebsd.org/D4561