Electron modem USART and buffer handling fixes

[avtolstoy]

Closes #1138 and #1104.

Fixes several issues:

• USART_FLAG_TXE was not checked when writing into DR register from system thread which caused an unsent data byte to be overwritten
• RX flow control was not correctly handled. When an internal ElectronSerialPipe buffer overflows, a data byte should be left in DR register to de-assert RTS line to ask modem to pause transmission for a while
• ElectronSerialPipe circular buffer was initialized to 1024 bytes, but the implementation allowed only 1023 bytes to be held inside the buffer
• AT+CLAC-like commands weren't parsed correctly (thanks @technobly)

Relevant tests:

• MDM_01_socket_writes_with_length_more_than_1023_work_correctly
• MDM_02_at_commands_with_long_response_are_correctly_parsed_and_flow_controlled

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Doneness:

• Contributor has signed CLA
• Problem and Solution clearly stated
• Code peer reviewed
• API tests compiled
• Run unit/integration/application tests on device
• Add documentation (N/A)
• Add to CHANGELOG.md after merging (add links to docs and issues)

Bugfixes

• [Electron] Fixed modem USART and buffer handling [Fixes #1138] [Fixes #1104]

[technobly]

@avtolstoy any comment on my question above? Let me know if you need me to verify tests on a particular electron.