Bug fix on partial message retrieval

[digant73]

BUG FIXES:

• wrong retrieval of partial message from L1 cache: this scenario typically happens when the TFT receives a burst of messages (e.g. the output for "M420 V1 T1"). The first fully received message (terminated by "\n") is enough to trigger the L1 cache as available (infoHost.rx_ok[portIndex] set to "true") for reading. As consequence, the partial retrieved message is not properly processed by the TFT (e.g. a partial echo message is displayed as an error popup, a truncated message breaks the expected handling etc...).
  E.g. the wrong number of rows sometimes displayed on Mesh Edit menu (see attached picture) (e.g. a 6x7 grid instead of the expected 7x7) was caused by this bug.
  Probably, the bug occurs more often on not fast TFTs and/or too fast/burst communication speed with main boards.
  The bug fix could fix some unusual bugs reported by users.

PR STATE: ready for merge