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>

util/lint/lint-000-license-headers

@@ -81,7 +81,7 @@ fi
 
 #get initial list from git, removing HEADER_EXCLUDED files.
 #make a copy to check for the old style header later.
-headerlist=$(${FIND_FILES} $HEADER_DIRS | egrep -v "($HEADER_EXCLUDED)")
+headerlist=$(${FIND_FILES} $HEADER_DIRS | grep -E -v "($HEADER_EXCLUDED)")
 
 #update headerlist by removing files that match the license string
 check_for_license() {