build-from-scratch: various cleanups

- Fix some typos, rephrase a number of sentences.

- Explain how to fix an out of date config-* file.

- Move LD_LIBRARY_PATH=alsa-lib into the ALSA section as opposed to
  interrupting some unrelated flow.

- Rename some section headers, add some in the toolchains section.

- Add numerous ``monospace`` backticks.

- Add missing space '#comments'

- Use VERBOSE=1 and make help in some examples - much easier to ignore
  or remove than to guess.

- PATH is already exported (otherwise it would not work).

- Replace deprecated backticks `pwd` with $(pwd).
  http://mywiki.wooledge.org/BashFAQ/082

- Replace `pwd`/../../etc with $XTENSA_ROOT which is shorter to type and
  compatible with make -C or ninja -C

- Convert one example to cmake -B && make -C to show the possibility.

Not done to keep this large patch somewhat reviewable in a
reasonable time: address the creeping platforms duplication.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

Author: marc-hb