samples/bluetooth/hci_pwr_ctrl stack overflow on nRF52DK_nRF52832

[cederom]

Describe the bug
Random stack overflow on CPU 0 jumps in when running samples / bluetooth / hci_pwr_ctrl on nRF52DK (nRF52832). It happens once per day.

To Reproduce
Steps to reproduce the behavior:

1. cd $ZEPHYR_BASE/samples/bluetooth/hci_pwr_ctrl
2. west build -b nrf52dk_nrf52832
3. Flash firmware.
4. Watch console logging output and observe random stack overflows :-)

Expected behavior
Sample application does not crash.

Impact
Sample program contains bug. I am not sure if this is setup (stack size build time configuration) or HCI Bluetooth stack bug.

Logs and console output

*** Booting Zephyr OS build zephyr-v2.4.0-2806-gdb5270bd933c  ***
Starting Dynamic Tx Power Beacon Demo
Bluetooth initialized
Dynamic Tx power Beacon started
Get Tx power level ->-> default TXP = 0
[00:00:00.270,904] <inf> bt_hci_core: HW Platform: Nordic Semiconductor (0x0002)
[00:00:00.270,904] <inf> bt_hci_core: HW Variant: nRF52x (0x0002)
[00:00:00.270,935] <inf> bt_hci_core: Firmware: Standard Bluetooth controller (0x00) Version 2.4 Build 99
[00:00:00.271,606] <inf> bt_hci_core: Identity: F1:E8:9E:D0:47:0E (random)
[00:00:00.271,636] <inf> bt_hci_core: HCI: version 5.2 (0x0b) revision 0x0000, manufacturer 0x05f1
[00:00:00.271,636] <inf> bt_hci_core: LMP: version 5.2 (0x0b) subver 0xffff

...

Get Tx power level -> TXP = 0
Set Tx power level to -3
Actual Tx Power: -4
Get Tx power level -> TXP = -4
Set Tx power level to -8
Actual Tx Power: -8
Get Tx power level -> TXP = -8
Set Tx power level to -15
Actual Tx Power: -16

...

[00:50:19.449,462] <err> os: ***** MPU FAULT *****
[00:50:19.449,462] <err> os:   Stacking error (context area might be not valid)
[00:50:19.449,462] <err> os: r0/a1:  0xffeffc9f  r1/a2:  0x20229f00  r2/a3:  0xfffaefdf
[00:50:19.449,493] <err> os: r3/a4:  0x82080041 r12/ip:  0xeeb7fad6 r14/lr:  0x20204101
[00:50:19.449,493] <err> os:  xpsr:  0x4c904800
[00:50:19.449,493] <err> os: Faulting instruction address (r15/pc): 0x2fe7dfe3
[00:50:19.449,493] <err> os: >>> ZEPHYR FATAL ERROR 2: Stack overflow on CPU 0
[00:50:19.449,493] <err> os: Current thread: 0x20000810 (unknown)
[00:50:20.150,115] <err> os: Halting system

Environment (please complete the following information):

• OS: FreeBSD 12.2-RELEASE-p1
• Toolchain: arm-none-eabi-gcc (GNU Arm Embedded Toolchain) 9.3.1 20200408 (release)
• Commit: db5270bd933cc2e7b222f93a791a9351157c3ce0 (HEAD -> master, origin/master)

Additional context
Crash always happens at that exact PC location.
Its nice to see my first Zephyr kernel crash :-) :-)

[nashif]

@carlescufi please assign

[cederom]

Thank you! :-)