Minor improvements of desktop build scripts.

Minor improvements of desktop build scripts.

This is a collection of small fixes I've recently made while playing
with chromium-efl build_desktop.sh script. Those are:
1. Improve argument parsing. Previous code needed 2 forks and a pipe to
   parse every single argument passed to the script. The same
   functionality can be achieved by only using shell built-in commands.
   It's more efficient and IMO easier to read.
2. Use integer comparison operations for variables which are supposed to
   have only integer values.
3. Clean-up script help message.
4. Use find instead of inefficient for loop to find files newer than
   jhbuild stamp.

Bug: http://107.108.218.239/bugzilla/show_bug.cgi?id=9215
Reviewed by: Antonio Gomes, Kamil Klimek

Change-Id: If390100b05a362f4ac33a63684210d7990533068
Signed-off-by: Piotr Tworek <p.tworek@samsung.com>