efi_selftest: check for RISC-V boot-hartid in FDT

On RISC-V check that the /chosen node has a boot-hartid property.

To run the test configure with CONFIG_CMD_BOOTEFI_SELFTEST=y and issue

    setenv efi_selftest device tree
    setenv serial# myserial
    bootefi selftest

If the test succeeds, it reports the boot-hartid, e.g.

    boot-hartid: 1

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