Revert "scripts: random_mac: remove bashism, should work with all shells"

This reverts commit 24022d721720708ce82b15681fffcccbd95da1f4.
Revert "scripts: random_mac: replace prefix with an officially 'locally_administered': 02:ca:fe"

This reverts commit 70f9e8cc95fc52da94d59a8ef6c4cde4ff40af96.
Revert "scripts: random_mac: replace function with a more portable variant, which also works on POSIX shells (not-bash)"

This reverts commit 82dc95d1f9a5c57d31229eb2d0f29aab5fe99768.