boards/lora-e5-dev: add documentation for riotboot_serial

[fjmolinas]

Contribution description

This PR makes riotboot-serial an option for the LoRa-E5-Dev board. It also does some simple improvements

• adds "Done Flashing" at end of flash scripts
• if shell and riotboot_reset are included then the bootloader command is included as well
• moves riotboot_serial to bin

Testing procedure

• riotboot_serial still works
• test/rng with USEMODULE=riotboot_reset show the bootloader cmd
• read the docs

Issues/PRs references

Depends on #17244, but can be split (I hope the doc PR will get in fast enough to not be needed)

[fjmolinas]

I think with the GPIO idea from #17247 riotboot_serial should be the default programmer for this BOARD, only flash the bootloader once and then no need for external devices (the stm32 bootloader requires an FTDI since it works on an USART not mapped to the USB-C port.

[Teufelchen1]

@fjmolinas still interested in getting this in?