Merge https://source.denx.de/u-boot/custodians/u-boot-riscv

- Kautuk's semihosting patch:
  move semihosting library from arm directory to common place and add
  RISC-V support
- Zong's Kconfig patch:
  use "imply" instead of "select" to allow user to decide if
  SPL_SEPARATE_BSS should be selected