[riscv] Perform a writability test before applying relocations

[riscv] Perform a writability test before applying relocations

If paging is not supported, then we will attempt to apply dynamic
relocations to fix up the runtime addresses.  If the image is
currently executing directly from flash memory, this can result in
effectively sending an undefined sequence of commands to the flash
device, which can cause unwanted side effects.

Perform an explicit writability test before applying relocations,
using a write value chosen to be safe for at least any devices
conforming to the JEDEC Common Flash Interface (CFI01).

Signed-off-by: Michael Brown <mcb30@ipxe.org>