[rtos] Add default constructor for Semaphore

[geky]

Currently Semaphore can not be instantiated without an explicit count as a constructor argument. This limits where Semaphores can be declared and requires explicit initialization in several annoying places, such as in member variables and SingletonPtr targets.

This adds a default count of 0, which has shown to be the most common initial value used for semaphores.

cc @0xc0170, @c1728p9

[geky]

@mbed-bot: TEST

HOST_OSES=windows
BUILD_TOOLCHAINS=GCC_ARM,ARM,IAR
TARGETS=K64F,LPC1768

[mbed-bot]

[Build 647]
SUCCESS: Building succeeded and tests were run! Be sure to check the test results

[0xc0170]

This limits where Semaphores can be declared

More details?

cc @bogdanm

[geky]

This mostly limits Semaphores being declared in naive data-structures. Properly handling initialization of C++ objects is annoying and often skimmed over. Normally this isn't a problem since classes are usually passed around by reference, but in this case Semaphores is primitive to be passed around by value.

For example, just try to create an array of Semaphores and you'll have a hard time:

Semaphore semas[SEMA_COUNT];

This initial motivation for this change was use in the SingletonPtr class (#2158, example) which would require a syntactical change to support non-default constructors:

SingletonPtr<Semaphore> sema;

Additionally, storing a Semaphore as a member variable requires explicit construction (example), which can get annoying.

All of these have workarounds, but providing a default constructor avoids the reoccuring issue.

[0xc0170]

Thanks for providing more info

LGTM

[geky]

no problem : )