Upgrade to bootloader UI and UX

[Alerymin]

Currently, the bootloader is really good but the UI can look complicated for new users

My idea would be not to change the bootloader itself but the UI and UX to make it more user friendly.

• Select slots to flash instead of letting flashing everything (to make Epsilon installation easier)
• Changing slots naming and write it explicitely
  • Slot 1 -> Slot A
  • Slot 3 -> Slot B
  • Slot 2 -> Slot EX (this is only an example)
• Displaying Slot 2 (Slot EX) below Slot 3 (Slot B)
• Not displaying empty/Invalid slots

Here are some concept arts: Full Bootloader

Bootloader with only Upsilon

Bootloader with Upsilon and Epsilon

Empty bootloader

Slots screen with all systems installed

Slots screen with no system installed

Flash screen for Slot A

Flash screen for Apps

Flash screen for All