package/grep: fix egrep/fgrep shebang

package/grep: fix egrep/fgrep shebang

egrep/fgrep are wrapper scripts, calling the grep binary with the correct
arguments.

The shell wrappers use the value of SHELL at build time as the shebang value
in these wrapper scripts, which in Buildroot points to /bin/bash.

The target may not have bash available, causing runtime errors.

As a fix, add a post-install hook to change this to /bin/sh.

If the target does not have /bin/sh, simply remove the aliases.

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>