"select_dtmcontrol", "select_dbus" and other variables should be per target

[JanMatCodasip]

Note to self: Discovered by code review that the following variables should be unique per each target (not globals shared by all RISC-V targets):

• select_dtmcontrol
• select_dbus
• select_idcode
• select_dbus

Reason: There can be different JTAG instruction lengths in each target (num_bits). See: https://github.com/riscv/riscv-openocd/blob/riscv/src/target/riscv/riscv.c#L454

The below variables (related to BSCAN tunneling, and currently also shared by all RISC-V targets) should be reviewed whether they need the same treatment:

• bscan_tunnel_type
• bscan_tunnel_ir_width
• bscan_tunnel_ir_id
• ir_user4
• bscan_tunneled_ir_width
• _bscan_tunnel_data_register_select_dmi
• _bscan_tunnel_nested_tap_select_dmi

See: https://github.com/riscv/riscv-openocd/blob/riscv/src/target/riscv/riscv.c#L53