More file operations of FatFs made available

• Additional file-related capabilities of FatFs available now:
  • File seeking in stdio and in the C++ standard library.
  • Directory operations rename(), mkdir() and unlink().
• Extended the smoke test run by the GitHub action with basic tests for file operations.

Synced with Circle Step 42

• Synced with Circle Step 42.
• Switched the FAT filesystem implementation to FatFs. While FatFs offers more features than Circle's FAT filesystem implementation that was used previously, this release of circle-stdlib only aims to be compatible with previous releases regarding filesystem-related functionality.

Sync with Circle Step 41.2

• Synced with Circle Step 41.2.

Sync with Circle Step 41.1

• Synced with Circle Step 41.1.
• Minor fix in the README.md file.

Sync with Circle Step 41

• Synced with Circle Step 41.
• Updated mbedtls to release 2.16.4.
• New configure option --qemu to build with support for QEMU semihosting mode.
• Added GitHub action that builds
  with the supported compilers in 32-bit and 64-bit mode and runs a smoke test under QEMU.
• Minor changes to circle-stdlib and mbedtls samples so they terminate under QEMU.

Updated newlib from upstream

• Updated circle-newlib from upstream to fix a compilation error in the C++ sample when using gcc 8.3-2019.03.

Sync with Circle Step 40

• Synced with Circle Step 40.
• Raspberry Pi 4 support.

Sync with Circle Step 39.2

• Synced with Circle Step 39.2.

Sync with Circle Step 39.1

• Synced with Circle Step 39.1.
• AArch64 support.
• Removed deprecated build script build.bash (use configure and make instead).

Sync with Circle Step 39 and mbed TLS 2.16.0

• Synced with Circle Step 39.
• Synced with mbed TLS 2.16.0.