efi_selftest: merge FDT and RISC-V tests

The test for the RISCV_EFI_BOOT_PROTOCOL retrieves the boot hart id via the
protocol and compares it to the value of the boot hart id in the device
tree. The boot hart id is already retrieved from the device tree in the FDT
test.

Merge the two tests to avoid code duplication.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>