profiles: Mask sys-boot/gnu-efi,refind[custom-cflags]

Both packages are very sensitive when it comes to compile flags which can easily
result in black screens, hangs or crashes. Also, the average joe should never
have any desire or advantage to set custom flags on those packages. Thus mask
the use flag.

profiles/base/package.use.mask

@@ -7,6 +7,16 @@
 # This file is only for generic masks. For arch-specific masks (i.e.
 # mask everywhere, unmask on arch/*) use arch/base.
 
+# Mike Gilbert <[email protected]> (09 Jul 2017)
+# Matthias Dahl <[email protected]> (05 Jul 2017)
+# Both are not your typical garden-variety Linux programs and are
+# rather sensitive when it comes to compiler flags, resulting in
+# black screens, hangs or crashes. The average joe should have no
+# need or even advantage to set custom compiler flags at all.
+# Example: https://bugs.gentoo.org/619628
+sys-boot/gnu-efi custom-cflags
+sys-boot/refind custom-cflags
+
 # Michał Górny <[email protected]> (8 Jul 2017)
 # The clutter backend is broken with new GTK+3 versions and upstream
 # haven't been able to fix it so far. Enforcing an old GTK+3 version