This code prevents the ARMC6 compiler/linker from removing
SUB_REALLOC/CALLOC symbols from image when LTO is enabled
Fixes below error:
L6137E: Symbol $Sub$$calloc was not preserved by the LTO codegen but is needed by the image.
Also includes:
* rename `mbed_sleep_manager.c` to `mbed_power_mgmt.c` to match its
header file
* create Doxygen groups for public and internal APIs
* use relative path to include header files where inconsistent
* update references to internal APIs throughout libraries
* update the copyright year for all modified files