target/riscv: early exit support for memory access operations

(1) Error code and 'skip_reason' string were replaced with memory access
    status. It allows to specify whether OpenOCD should exit the access
    early.
(2) Slightly refactored 'read_memory' and 'write_memory' functions.

Checkpatch-ignore: MACRO_ARG_PRECEDENCE, MULTISTATEMENT_MACRO_USE_DO_WHILE
Checkpatch-ignore: TRAILING_SEMICOLON
Signed-off-by: Farid Khaydari <f.khaydari@syntacore.com>