cmd: bootefi: merge efi_install_fdt() and efi_process_fdt()

This is a preparatory patch for reworking do_bootefi() in later patch.
For simplicity, merge two functions.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>

Return error code of efi_install_configuration_table() when aborting from
efi_install_fdt().

Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>