[EGD-6678] Fix Display backlight turns on when is set off

Duty cycle update performed only on enabled pwm

[EGD-6651] Fix communication with modem on Linux

Fixed cellular bsp on Linux, so now emulator is able
to comunicate with modem. Fixed memory leak in CellularResult struct.

[EGD-6319] Refactor cellular modem directory

- Removed not used files from old implementations.
- Renamed some classes to follow naming convention.
- Renamed methods to use consistent styling.
- Restructured code.

[EGD-6591] Remove excessive logs from USB stack

Removed log entries in conflict with charger driver BSP logs

[EGD-6619] Fix Display backlight switches periodically

Reload pwm duty cycle value after CPU frequency change

[EGD-5748] Remake Cellular flow

This commit remakes cellular data flow
introducing full error propagation.

[EGD-6457] Add power management for USB

When the USB is disconnected, we turn off the peripherals
by going into the suspend bus mode.

[EGD-6355] Adjust MuditaOS to be used with Secure Boot

The start address of the OS image adjusted to comply with HAB
Secure Boot. Proper SNVS initialization performed at system startup.
SNVS drivers updated to most recent version. Image signing can be
performed as a part of build process. See doc/secure_boot.md for
more info.

[EGD-6545] Enable magnetometer LPDCM mode

To be able to reduce the power usage of the phone, magnetometer
has been switched into interrupt driven LPDCM mode. Added some
fixups maintaining current behavior on startup and disabling
the tethering mode

[EGD-6499] Fix audio initialization

Now the peripherals and the audio clocks will be turned on
only when the user is using the audio, which will save energy.

[EGD-6506] Turn on function keys backlight during call

Backlight of function keys are on when call is active.
When call ends, previous backlight mode is restored.

[EGD-6118] BT low power initial implementation

The `Pure`'s power consumption was measured in three variants:
 * BT off
 * BT on (master)
 * BT on (EGD-6118)

Measurement results (respectively):
* idle [mA]:
 ** 78-81
 ** 83-85
 ** 78-81

* BT headphones connected [mA]
 ** -
 ** 85-87
 ** 82-85

* music play [mA] (may be not precise)
 ** 90-100
 ** 98-105
 ** 101-115

[EGD-6431] Add battery charging temperature ranges

Added actions of several cell temperature
ranges according to requirements.

[EGD-6515] Answer/reject calls using headset

Clean up propagation of headset keys events
Clean up volume control with headset keys
Update key mappings for simulator
Use headset OK key to answer/reject calls

[EGD-6505] Implement “On when active” Keypad light functionality

Implement “On when active” Keypad light functionality

[EGD-5360] Add USB current selection

Charging current selection algorithm
depending on USB type.

[EGD-6214] Add slider position reading after disabling tethering

To be able to set proper phone mode changed by the slider during
tethering, the forced readout of slider position has been added

[EGD-6416] Add handling of headset keys

Change the volume level using headset keys

[EGD-5748] Revert remake cellular flow

Reverts changes due to reported issues

[EGD-6425] Update USB submodule version

Restored USB submodule version to HEAD