uart: BW improvements (#4620)

* uart fixes and BW improvements

* uart: read_char straightly use hw buffer

* +attributes for functions called by ISR

* uart: BW improvements
read_char straightly use hw buffer (+ ~10%bw)
read by block (+ ~190%bw) (instead of generic Stream::readBytes)
attributes for functions called by ISR
remove overrun message
remove some ISR flags which were not honoured

* fix merge

* fix buffer overflow

* serial stress test sketch

* astyle

* serial stress example: interactive keyboard, stop reading, overrun

* serial device test: bandwidth & overrun

* update + HardwareSerial::hasError()

* interactive overrun in example

* astyle

* Test using @plerup's SoftwareSerial as submodule (tag 3.4.1)

* update upstream ref (fix warning)

* host mock uart/read(buf,size)

* reset style changes in submodules before style diff

* update build_boards_manager_package.sh for submodules

* trigger CI (removing space)

* cannot reproduce locally the CI issue, setting bash -x option to get live trace

* remove previously added (in this PR) 'set -e' in package builder (passes local tests, not real CI)
script-comment new recipe.hooks.core.prebuild.3 (along with already commented .1 and .2)
moved CI package test to be first on the test list
remove 'set -x', wish me luck

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "tools/sdk/ssl/bearssl"]
 	path = tools/sdk/ssl/bearssl
 	url = https://github.com/earlephilhower/bearssl-esp8266
+[submodule "libraries/SoftwareSerial"]
+	path = libraries/SoftwareSerial
+	url = https://github.com/plerup/espsoftwareserial.git