Kbuild: Ignore GREP_OPTIONS env variable

Kbuild: Ignore GREP_OPTIONS env variable

When building the kernel in a shell which defines GREP_OPTIONS so that
grep behavior is modified, we can break the generation of the syscalls
table like so:

__SYSCALL_COMMON(^[[01;31m^[[K0^[[m^[[K, sys_read, sys_read)
__SYSCALL_COMMON(^[[01;31m^[[K1^[[m^[[K, sys_write, sys_write)
__SYSCALL_COMMON(^[[01;31m^[[K1^[[m^[[K0, sys_mprotect, sys_mprotect) ...

This is just the initial breakage, later we barf when generating
modules.

In this case, GREP_OPTIONS contains "--color=always" which adds the shell
colors markup and completely fudges the headers under ...generated/asm/.

Fix that by unexporting the GREP_OPTIONS variable for the whole kernel
build as we tend to use grep at a bunch of places.

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Michal Marek <mmarek@suse.cz>