[sb2] Replace calls to deprecated fgrep, egrep and cleanup. JB#59434

`egrep` and `fgrep` are deprecated, use `grep -E` or `grep -F` respectively.
Replace `$(...)`  notation instead of legacy backticks.
Avoid useless `cat` where `read` can do the job better.

Signed-off-by: Björn Bidar <bjorn.bidar@jolla.com>

Author: Thaodan