Update HeapBlockDevice example

[kegilbert]

Small example in the HeapBlockDevice header file was not valid, added size parameters to program and read calls.

@geky Wasn't sure who was the best person to put on here was, figured you were closest to filesystem changes at the moment and last touched this file. ¯/_(ツ)_/¯ Feel free to change the reviewer to someone else.

[geky]

Ah, actually this example has definitely become a bit rotten.

• Needs to call bd.erase before calling bd.program
• How about we just #define BLOCK_SIZE at top? Then we can use BLOCK_SIZE for the erase/read/program calls

[kegilbert]

@geky: Updated. Should I erase the whole allocated block object or is just the block I'm using in the example fine?

[geky]

Looks good 👍, just need to erase the first block. Think of a write as erase followed by program.

[mbed-ci]

Build : SUCCESS

Build number : 81
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/5292/

Triggering tests

/test mbed-os

[mbed-ci]

Test : SUCCESS

Build number : 23
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/5292/23

[0xc0170]

retest uvisor

[mbed-ci]

Test : SUCCESS

Build number : 41
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/5292/41