target/cortex-m: enable C_DEBUGEN during examine

target/cortex-m: enable C_DEBUGEN during examine

Current code for Cortex M does not set C_DEBUGEN as soon as
possible, (which means during target examine), but later-on either:
1) at command 'halt' (e.g. for 'gdb-attach' event);
2) at command 'soft_reset_halt';
3) at commands 'reset', 'reset halt' or 'reset init';
4) during polling, but only if the target:
    = enter in 'double fault', or
    = exit from a reset, or
    = halts (not possible if C_DEBUGEN is not set)

Plus, if commands in 1) or 2) are executed before the very first
poll of the target, the value of 'cortex_m->dcb_dhcsr' is used not
initialized while writing it back in DCB_DHCSR.

Another side effect of this situation is that it's possible to set
a HW breakpoint with the target running, while C_DEBUGEN is not
set. Accordingly to [1], C1.3.1 "Debug authentication":
    When DGBEN is LOW and DHCSR.S_HALT == 0:
    ...
    FPB breakpoints do not generate an entry to Debug state and,
    if no DebugMonitor exception is generated, will escalate to
    HardFault, Lockup, or be ignored.
On STM32MP15x I get HW breakpoint ignored, while on STM32F411 I
get HardFault.
E.g. following these steps:
- power-on a pre-flashed board that starts running the firmware;
- connect openocd, without halting or resetting the board;
- set a HW breakpoint to some address often executed;
- wait, but the board doesn't halt ...;
- type the command 'halt';
- if the Cortex-M has HardFault it would be visible and the fault
  is at the breakpoint address;
- if no HardFault then type the command 'resume';
- wait and the board will finally halt at the HW breakpoint.

A similar issue has been detected on Cortex-A code and fixed by
commit bff87a7f28fb ("target/cortex_a: enable DSCR_HALT_DBG_MODE
during examine").
Follow the same approach and set C_DEBUGEN during examine.
Also, initialize 'cortex_m->dcb_dhcsr' during examine.

[1] ARM DDI 0403E "ARM v7-M Architecture Reference Manual"

Change-Id: I5b0b23403634f7dfce38f104bba9f59c33eb3e99
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5702
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Moritz Fischer <moritzf@google.com>