cortex_m: fix scan-build false positive

Scan-build is unable to detect that 'target->dbg_msg_enabled' does
not change across the function cortex_m_fast_read_all_regs().
It incorrectly assumes that it can be false at the first check (so
'dcrdr' get not assigned) and it is true later on (when 'dcrdr'
get used).
This triggers a false positive:
	src/target/cortex_m.c:338:12: warning:
		3rd function call argument is an uninitialized value
		[core.CallAndMessage]
	retval = mem_ap_write_atomic_u32(armv7m->debug_ap, DCB_DCRDR, dcrdr);

Use a local variable for 'target->dbg_msg_enabled' so scan-build
can track it as not modified.
While there, change the type of 'target->dbg_msg_enabled' to
boolean as there is no reason to use uint32_t.

Change-Id: Icaf1a1b2dea8bc55108182ea440708ab76396cd7
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8391
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/target/target.c

@@ -5813,7 +5813,7 @@ static int target_create(struct jim_getopt_info *goi)
 	}
 
 	target->dbgmsg          = NULL;
-	target->dbg_msg_enabled = 0;
+	target->dbg_msg_enabled = false;
 
 	target->endianness = TARGET_ENDIAN_UNKNOWN;