core: use GRUB_TERM_ definitions when handling term characters

* Also use hex value for GRUB_TERM_ESC as '\e' is not in the C standard and is not understood by some compilers

grub-core/tests/cmdline_cat_test.c

@@ -103,7 +103,7 @@ cmdline_cat_test (void)
 					     '/', 't', 'e', 's', 't', '.',
 					     't', 'x', 't', '\n',
 					     GRUB_TERM_NO_KEY,
-					     GRUB_TERM_NO_KEY, '\e'},
+					     GRUB_TERM_NO_KEY, GRUB_TERM_ESC},
 					 23);
 
       grub_video_checksum ("cmdline_cat");