docs: clarify comp's 'option userinit yes'

It's ignored if the option 'userspace' is not set.
comp: warn if the user sets userinit but not userspace

The option 'userinit' is ignored if the option 'userspace' is not set,
so warn if the .comp specifies it.
comp: fix a couple of syntax errors when using userinit

This fixes two silly bugs in comp.  I guess userinit is not widely used...

Thanks to Curtis Dutton for the bug report and bug fix.
bldc: remove unused option userinit