Bugfix: LPC1768 IAP could not copy flash to flash

[multiplemonomials]

Summary of changes

This fixes an issue reported on the forums here: https://forums.mbed.com/t/limitation-of-flash-iap-for-lpc176x-flashing-from-ram-only/19892

The LPC1768's FlashIAP implementation could not successfully copy flash data to flash. Now, I adapted the existing RAM buffer scheme in the driver to fix this issue. I also did a general cleanup of the lpc1768 flash code, including replacing the magic 1024 number with the page size, and fixing the reported deadlock issue if an operation fails (missed core_util_critical_section_exit() calls).

Impact of changes

Migration actions required

Documentation

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

See comment below

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[multiplemonomials]

Ran tests:

96% tests passed, 3 tests failed out of 72

Total Test time (real) = 1435.25 sec

The following tests did not run:
	  8 - mbed-hal-crc (Skipped)
	 11 - mbed-hal-lp-ticker (Skipped)
	 14 - mbed-hal-ospi (Skipped)
	 16 - mbed-hal-qspi (Skipped)
	 27 - mbed-hal-trng (Skipped)
	 32 - mbed-hal-verify-arduino-pinmap (Skipped)
	 43 - mbed-platform-crash-reporting (Skipped)
	 53 - mbed-platform-stats-cpu (Skipped)
	 54 - mbed-platform-stats-heap (Skipped)
	 55 - mbed-platform-stats-sys (Skipped)
	 56 - mbed-platform-stats-thread (Skipped)
	 71 - mbed-connectivity-nfc-eeprom (Skipped)

The following tests FAILED:
	  5 - mbed-usb-device-serial (Failed)
	 65 - mbed-rtos-mutex (Failed)
	 67 - mbed-rtos-semaphore (Failed)

Note that the new flash test is passing!