configs/releng/build.sh:
Instead of downloading UDK2018 based edk2 UEFI shell, use the Shell
binaries from the edk2-shell package.
This effectively makes the use of EFI shell v1 obsolete.

configs/releng/efiboot/loader/entries/uefi-shell-v*:
Renaming UEFI shell entries to match the packaged edk2-shell based UEFI
shell binaries.

configs/releng/build.sh:
Renaming the systemd-boot bootloader entries for UEFI shell (normal and
full version) to reflect their file names, which are derived from the
binary names in the edk2-shell package.
Copying the edk2-shell based UEFI shell binaries for normal efiboot and
.iso based "el-torito" efiboot mode.

docs/README.build:
Adding edk2-shell to the requirements in the documentation, as the
package is now used to provide UEFI shell.