tree: Replace egrep with grep -E

For compatibility reasons, egrep is just a wrapper around grep today.
Thus, replace it with `grep -E`.

Change-Id: Ief08a22e4cd7211a3fee278492c95d37f9e058fa
Signed-off-by: Felix Singer <felix.singer@secunet.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74171
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>

payloads/libpayload/curses/PDCurses/aclocal.m4

@@ -463,7 +463,7 @@ EOF
 			DYN_COMP=""
 		else
 			slash="\\"
-			mh_dyncomp="`egrep -c $slash$a conftest.tmp`"
+			mh_dyncomp="`grep -E -c $slash$a conftest.tmp`"
 			if test "$mh_dyncomp" = "0"; then
 				DYN_COMP="$a -DDYNAMIC"
 				AC_MSG_RESULT($a)