proposal: update prompt message of `build_support/packages.sh`

[mooxiu]

When preparing building the project and execute build_support/packages.sh, the prompt message will be:
Proceed? [Y/n] :

The default behavior (when pressing enter/return without inputting y or n) is do nothing, however, the common practice is that the default behavior should be the same with the capitalized option (in this case, Y). If someone assume default behavior to be yes, then it might lead to lack of necessary packages, which is really confusing.

My proposal, change this line from:

        read -p "Proceed? [Y/n] : " yn

to:

        read -p "Proceed? [y/N] : " yn

[skyzh]

feel free to submit a pull request