bsim_bt: compile.sh: Use lowercase for internal vars

This makes it possible to tell at a glance which variables are internal
to the script and which ones are parameters to it, which is very
helpful.

This convention is pretty common. See e.g. Google's shell style guide at
https://google.github.io/styleguide/shell.xml#Naming_Conventions, and
https://github.com/icy/bash-coding-style#naming-and-styles

. It's older
than those though.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>