Add back wrongly removed '\n' K-V pair sent to the device

[PrzemekWirkus]

Changes

Align how DUT and host together send and receive K-V messages between each other.

• Together with Changing the key-value pair regex to include searches for new line #104 should significantly improve e.g, mbed_drivers::echo tests and DAPLink UX
• While sending (TXD) K-V from htrun logger will not print white-space characters new line at the end
• Updated README to reflect missing \n in K-V format
• Note that greentea-client will always send newline character to host, see here.

Testting

mbedgt: test suite report:
+--------------+---------------+-------------------------+--------+--------------------+-------------+
| target       | platform_name | test suite              | result | elapsed_time (sec) | copy_method |
+--------------+---------------+-------------------------+--------+--------------------+-------------+
| K64F-GCC_ARM | K64F          | tests-mbed_drivers-echo | OK     | 23.73              | shell       |
+--------------+---------------+-------------------------+--------+--------------------+-------------+
mbedgt: test suite results: 1 OK
mbedgt: test case report:
+--------------+---------------+-------------------------+------------------+--------+--------+--------+--------------------+
| target       | platform_name | test suite              | test case        | passed | failed | result | elapsed_time (sec) |
+--------------+---------------+-------------------------+------------------+--------+--------+--------+--------------------+
| K64F-GCC_ARM | K64F          | tests-mbed_drivers-echo | Echo server: x16 | 1      | 0      | OK     | 1.65               |
| K64F-GCC_ARM | K64F          | tests-mbed_drivers-echo | Echo server: x32 | 1      | 0      | OK     | 3.19               |
| K64F-GCC_ARM | K64F          | tests-mbed_drivers-echo | Echo server: x64 | 1      | 0      | OK     | 6.29               |
+--------------+---------------+-------------------------+------------------+--------+--------+--------+--------------------+
mbedgt: test case results: 3 OK
mbedgt: completed in 23.82 sec

[PrzemekWirkus]

@mazimkhan @adbridge @ConorPKeegan FYI

[bridadan]

Tested locally for K64F and GCC_ARM,ARM, and IAR. Prevents bad state from echo test from ocurring and no regressions on other tests!

+1