ArmVirtPkg/PlatformBootManagerLib: register boot options and hot keys

ArmVirtPkg/PlatformBootManagerLib: register boot options and hot keys

Register the Enter key as the continue key (hot key to skip the boot
timeout). Map the F2 and ESC keys to the UI. Register the memory-mapped
Shell boot option.

The patch parallels OvmfPkg commit 07dd96e82032. The
PlatformRegisterFvBootOption() and PlatformRegisterOptionsAndKeys()
functions are copied almost verbatim. The only changes are: internal
linkage for these functions (i.e., STATIC), and mentioning the ESC key in
the comments.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Ruiyu Ni <ruiyu.ni@Intel.com>