grub2: force -fno-stack-protector in CFLAGS

grub2 fails to configure when BR2_SSP_ALL is enabled, with the following
configure error:

  checking whether -fno-asynchronous-unwind-tables works... yes
  checking whether -fno-unwind-tables works... yes
  checking for target linking format... unknown
  configure: error: no suitable link format found

This can be worked around by enforcing -fno-stack-protector in the
package CFLAGS in a way that overrides the SSP flag, as is already done
for the valgrind package.

Fixes bug #10261.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Reported-by: Dr I J Ormshaw <ian_ormshaw@waters.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>