Allow various configuration options to be per-hart

[timsifive]

Quoting Jan Matyas' feedback on http://openocd.zylin.com/#/c/5821/6/src/target/riscv/riscv.c:

I believe we should store the ebreakm, ebreaks, ..., ... and most other flags separately for each RISC-V target -- there can be more of them in the JTAG chain, each possibly with different settings.

Command "riscv " would apply to the currently selected target.

Command "<target_name> riscv " would apply to the specified target.

To get the target the command applies to inside the handler, use get_current_target(), as is already used in "dmi_read" and "dmi_write" command handlers.

IMO, these flags that should be stored separately per each target:

• riscv_ebreak*
• riscv_enable_virt2phys
• ir_*
• custom exposed csrs
• other custom exposed registers
• riscv_enable_virtual
• riscv_prefer_sba

Timeout related settings can probably be specified just globally for all RISC-V targets.

As for the BSCAN bridge options, I am not sure if these are global or per-target (not sure how BSCAN is utilized in this specific implementation).