Remove the static allocation for the dirent, and allocate it from the
heap during opendir().
Removing the static data can reduce RAM usage on some toolchains when
directories are not being used. The static allocation sometimes is
combined with the file handle array and can't be dropped by the linker.
Original readdir() was not thread-safe at all.
This was in violation of POSIX which states the result of readdir "is
not overwritten by another call to readdir() on a different directory
stream."
POSIX also defines readdir_r() as separate totally reentrant form where
the caller allocates the dirent, but this is generally deprecated as it
opens the door for an inadequate allocation causing a stack smash. Full
reentrancy is not typically necessary - having readdir()'s buffer data
be per-DIR is generally sufficient.
Mbed OS will soon be changing directory structure to the below:
```
[component name]
├── mbed_lib.json
├── CMakeList.txt
├── README.md
├── include
│ └── [component name]
│ └── internal
├── source
└── tests
├── <framework>
│ └── <test suite one>
├── UNITTESTS
│ └── <unit test suite one>
└── TESTS
└── <greentea test suite one>
```
This PR implements the above new directory structure for the current \platform directory.
**NOTE:** This PR does NOT include the refactoring of UNITTESTS as shown in the above directory structure. This will be done in a subsequent PR.
Kevin Bracey
Martin Kojtal
mbedmain
Marc Emmers
pea-pod
Paul Szczeanek
Ashok Rao