boot/shim: bump to version 15.4

boot/shim: bump to version 15.4

- Use the tarball provided by upstream developers instead of the one
  generated by Github. Indeed
  https://github.com/rhboot/shim/releases/tag/15.4 indicates "As
  usual, please use the shim-15.4.tar.bz2 tarball, rather than the
  other two archives github automatically produces."

- The tarball now includes the gnu-efi code, so we no longer need to
  select gnu-efi and have it as a build dependency. We continue to use
  BR2_PACKAGE_GNU_EFI_ARCH_SUPPORTS as we still only build for those
  architectures that have gnu-efi support. We also drop the
  EFI_INCLUDE, EFI_PATH and LIBDIR variables, as gnu-efi no longer
  needs to be searched in STAGING_DIR.

- Drop all four patches, which were backports from upstream.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>