add get_default_instance

[maclobdell]

Override the default "weak" implementation of the "get_default_instance" in the blockdevice class. Pass a pointer to the SDIO blockdevice instance. This allows applications to get the default block device without knowledge of the underlying type of storage.

Application example code:

BlockDevice *bd = BlockDevice::get_default_instance();

documentation of this feature:
https://os.mbed.com/docs/mbed-os/v5.13/apis/blockdevice.html

[maclobdell]

By the way, with this change in place, I ran the file system integration tests from the 'sip-workshop' branch of mbed-os. Several tests pass, but a few with large buffer sizes fail. They tests seem to crash and timeout for those.

|-------------------|---------------|-------------------------------|-----------------------------------------|--------|--------|---------|--------------------|
| LPC55S69_NS-ARMC6 | LPC55S69      | tests-integration-fs-single   | FLASHIAP+LFS 1 file, buff     1         | 1      | 0      | OK      | 6.65               |
| LPC55S69_NS-ARMC6 | LPC55S69      | tests-integration-fs-single   | FLASHIAP+LFS 1 file, buff     4         | 1      | 0      | OK      | 3.76               |
| LPC55S69_NS-ARMC6 | LPC55S69      | tests-integration-fs-single   | FLASHIAP+LFS 1 file, buff    16         | 1      | 0      | OK      | 3.04               |
| LPC55S69_NS-ARMC6 | LPC55S69      | tests-integration-fs-single   | FLASHIAP+LFS 1 file, buff    64         | 1      | 0      | OK      | 2.7                |
| LPC55S69_NS-ARMC6 | LPC55S69      | tests-integration-fs-single   | FLASHIAP+LFS 1 file, buff   256         | 1      | 0      | OK      | 2.8                |
| LPC55S69_NS-ARMC6 | LPC55S69      | tests-integration-fs-single   | FLASHIAP+LFS 1 file, buff  1024         | 0      | 0      | ERROR   | 0.0                |
| LPC55S69_NS-ARMC6 | LPC55S69      | tests-integration-fs-single   | FLASHIAP+LFS 1 file, buff  4096         | 0      | 0      | SKIPPED | 0.0                |
| LPC55S69_NS-ARMC6 | LPC55S69      | tests-integration-fs-single   | FLASHIAP+LFS 1 file, buff 16384         | 0      | 0      | SKIPPED | 0.0                |
| LPC55S69_NS-ARMC6 | LPC55S69      | tests-integration-fs-single   | FLASHIAP+LFS format                     | 1      | 0      | OK      | 3.5                |
| LPC55S69_NS-ARMC6 | LPC55S69      | tests-integration-fs-threaded | FLASHIAP+LFS 2 files, buff 4b/256b      | 1      | 0      | OK      | 6.27               |
| LPC55S69_NS-ARMC6 | LPC55S69      | tests-integration-fs-threaded | FLASHIAP+LFS 3 files, buff 256b/1kb/4kb | 0      | 0      | ERROR   | 0.0                |
| LPC55S69_NS-ARMC6 | LPC55S69      | tests-integration-fs-threaded | FLASHIAP+LFS format                     | 1      | 0      | OK      | 2.37               |