NeuInk 0.2.0

Adds switching to VARAGH, CrossInk or CrossPoint from the settings, and repairs online
firmware downloads, which did not work in 0.1.0. A reader on 0.1.0 cannot download this
release itself: copy firmware-x4-pro.bin to the SD card and use Settings → System →
SD Card Firmware Update once. Online updates work from 0.2.0 on.

• Fixed online firmware downloads, including NeuInk's own updates. The reader's
  wolfSSL was built without SHA-384, so it could not verify the certificate chain
  of github.com and the download stopped with "Update failed" right after you
  confirmed it. SHA-384 is now enabled, and the build refuses to compile without
  it. NeuInk 0.1.0 has this fault, so a 0.1.0 reader cannot install this fix
  online: use SD Card Firmware Update once.
• Fixed a second reason online firmware downloads failed, with the bar showing the
  size and stopping at 0 bytes. The reader does not set its system clock at boot, so
  the certificate date checks of the pinned download ran against 1970 and wolfSSL
  could not even load the pinned root. The download now syncs the clock first, with
  the same NTP step KOReader sync uses.
• Added scripts/wolfssl_pin_check/check.sh, which handshakes with the pinned
  GitHub hosts using the reader's own wolfSSL source and settings. Run it before a
  release and whenever GitHub changes its certificates.
• Added Settings → System → Switch to Another Firmware. It installs the latest
  VARAGH, CrossInk or CrossPoint release over WiFi, after a warning that says SD card
  files and books should stay safe but a backup is sensible. It reuses the OTA updater
  and the pinned GitHub certificates, matches each project's own release asset name
  for the device, and refuses a release that has no SHA-256 digest.