Minor adjustments to embed_dtb.sh and make_dtbh.sh

[bcr]

Summary

This patch does the following:

embed_dtb.sh:

• Instead of using ls -l and parsing the output simply to get the file size, use stat(1) to get it. The ls output is intended for human consumption and may differ depending on the execution environment (i.e. human readable numbers).
• Replace backticks with $()
• Replace $var with ${var}
• Double quote vars to prevent word splits and globbing

make_dtbh.sh:

• Replace backticks with $()
• Replace $var with ${var}
• Double quote vars to prevent word splits and globbing

Test Plan

Run the scripts (especially embed_dtb.sh for the new way to determine the filesize) and check the result.