semihosting: User defined operation, Tcl command exec on host

Enabling a portion (0x100 - 0x107) of the user defined semihosting
operation number range (0x100 - 0x1FF) to be processed with the help of
the existing target event mechanism, to implement a general-purpose Tcl
interface for the target available on the host, via semihosting
interface.

Example usage:
- The user configures a Tcl command as a callback for one of the newly
	defined events (semihosting-user-cmd-0x10X) in the configuration
	file.
- The target can make a semihosting call with <opnum>, passing optional
	parameters for the call.

If there is no callback registered to the user defined operation number,
nothing happens.

Example usage: Configure RTT automatically with the exact, linked
control block location from target.

Signed-off-by: Zoltán Dudás <zedudi@gmail.com>
Change-Id: I10e1784b1fecd4e630d78df81cb44bf1aa2fc247
Reviewed-on: https://review.openocd.org/c/openocd/+/6748
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/target/semihosting_common.h

@@ -75,8 +75,14 @@ enum semihosting_operation_numbers {
 	SEMIHOSTING_SYS_WRITE = 0x05,
 	SEMIHOSTING_SYS_WRITEC = 0x03,
 	SEMIHOSTING_SYS_WRITE0 = 0x04,
+	SEMIHOSTING_USER_CMD_0x100 = 0x100, /* First user cmd op code */
+	SEMIHOSTING_USER_CMD_0x107 = 0x107, /* Last supported user cmd op code */
+	SEMIHOSTING_USER_CMD_0x1FF = 0x1FF, /* Last user cmd op code */
 };
 
+/** Maximum allowed Tcl command segment length in bytes*/
+#define SEMIHOSTING_MAX_TCL_COMMAND_FIELD_LENGTH (1024 * 1024)
+
 /*
  * Codes used by SEMIHOSTING_SYS_EXIT (formerly
  * SEMIHOSTING_REPORT_EXCEPTION).