Explore additional methods of flashing

[lpalgarvio]

Besides sysupgrade there's other methods of flashing devices.
Some of these might not be possible or practical to automate in the role.
Research into possibilities of automation within reasonability.
Nonetheless, some documentation or links can be added in the README.md.

Intro:

• https://openwrt.org/docs/guide-user/installation/start
• https://openwrt.org/docs/guide-user/installation/generic.flashing
• https://openwrt.org/docs/guide-user/installation/installation_methods/start

Sysupgrade (already implemented):

• https://openwrt.org/docs/guide-user/installation/sysupgrade.cli
• https://openwrt.org/docs/guide-user/installation/installation_methods/sysupgrade
• https://openwrt.org/docs/techref/sysupgrade

TFTP/FTP:

• https://openwrt.org/docs/guide-user/installation/installation_methods/tftp_generic
• https://openwrt.org/docs/guide-user/installation/generic.flashing.tftp
• https://openwrt.org/docs/guide-user/installation/generic.flashing.ftp

Serial:

• https://openwrt.org/docs/guide-user/installation/generic.flashing.serial
• https://openwrt.org/docs/guide-user/installation/installation_methods/serial
• https://openwrt.org/docs/techref/hardware/port.serial
• https://openwrt.org/docs/techref/hardware/port.serial.cables

JTAG:

• https://openwrt.org/docs/guide-user/installation/installation_methods/jtag
• https://openwrt.org/docs/techref/hardware/port.jtag