mk: shell: Guard a SHELL hack for cygwin with conditionals
mk: shell: Guard a SHELL hack for cygwin with conditionals

Also add detailed explanation of the workaround.

This fixes 'Bad substitution' error occurring when using Dash as a
shell on Debian-based systems.

embox/embox,embox/embox,Kakadu/embox,mike2390/embox,Kakadu/embox,embox/embox,Kakadu/embox,embox/embox,mike2390/embox,mike2390/embox,mike2390/embox,Kakadu/embox,mike2390/embox,embox/embox,embox/embox,Kakadu/embox,mike2390/embox,Kakadu/embox,Kakadu/embox,mike2390/embox