scpi-99 6.2.3.4 and 6.2.6 suggest returning multiple blocks is valid, so SCPI_ResultArbitraryBlockHeader() needs to insert a delimiter,
other changes are: accepting _GLIBCXX_HAVE_STDBOOL_H, testing for non-NULL data in writeData(), and some spelling fixes.

add SCPI_HelpQ callback , optional .description strings and provide conditional initializer macros dependent on config settings for USE_COMMAND_TAGS and USE_COMMAND_DESCRIPTIONS.

introduce search string to narrow down help output

…lt disabled

* Properly integrate command descriptions into examples. Preliminary descriptions added after consulting scpi-99 standard, as well as looking at the DMM and TEST handler source. Command paramater types hinted as <param> where appropriate. Return values or ranges given at the end of each description in round brackets.
* Default changed to USE_COMMAND_DESCRIPTIONS 0 to avoid unexpected impact on code side. Set to 1 to use.
* scpi_command_t defintion helper macros moves to types.h

* fixes some inaccuracies in scpi-def
* clean-up in utils.c
* add documentation to introduced functions
* correct CHAR_TO_LOWER

Allow makefile to be used directly for cross-compiling (e.g. when treated as a git submodule). This requires CC and AR to be passed for a particular toolchain, but CFLAGS must also specify CPU and FPU options -> introduce MODE variable. CFLAGS cannot be passed without need to override in the makefile, so a new variable is needed.

Cortex-M7 Example (linker: library and project use of VFP registers must match):
CC=arm-none-eabi-gcc AR=arm-none-eabi-ar MODE="-mcpu=cortex-m7 -mthumb -mfpu=fpv5-sp-d16 -mfloat-abi=hard"

(commit can be cherry-picked)

When compiled with USE_HELP_FILTER 1, HELP? accepts an optional search string -> HELP? command description extended to reflect input and output types.

remove / comment currently unused code. Double-indirection helper definition moved to utils.h.

This reverts commit c394eb9.