config.tftpRoot should handle relative paths properly

[thehonker]

with ~/Documents/tftpboot: (should resolve ~ properly)

╭─epers@skylight ~/Documents/tsbootkit
╰─➤  tsbootkit-pxed --config config.yaml
[00:20:46] pxed info: Loaded config from config.yaml: 0 reservation(s)
[00:20:46] pxed info: Starting PXE daemon on en6 (mode=dhcp, boot=pxelinux.0, tftp=~/Documents/tftpboot)
Fatal: Root directory does not exist: /Users/epers/Documents/tsbootkit/~/Documents/tftpboot

---

with /tftpboot: (works as expected)

╭─epers@skylight ~/Documents/tsbootkit
╰─➤  tsbootkit-pxed --config config.yaml
[00:21:08] pxed info: Loaded config from config.yaml: 0 reservation(s)
[00:21:08] pxed info: Starting PXE daemon on en6 (mode=dhcp, boot=pxelinux.0, tftp=/tftpboot)
Fatal: Root directory does not exist: /tftpboot

---

with ./tftpboot: (works as expected)

╭─epers@skylight ~/Documents/tsbootkit
╰─➤  tsbootkit-pxed --config config.yaml
[00:23:34] pxed info: Loaded config from config.yaml: 0 reservation(s)
[00:23:34] pxed info: Starting PXE daemon on en6 (mode=dhcp, boot=pxelinux.0, tftp=./tftpboot)
Fatal: Unknown network interface: en6