package/grep: fix egrep/fgrep shebang

egrep/fgrep are using autotools SHELL variable to execve the grep executable with correct options, shell is defaulted to /bin/bash which we usually don't have on target.

---
I tested manually changing the package to:

GREP_CONF_OPTS = SHELL=/bin/dash --exec-prefix=/

And this works. If /bin/sh is a link to /bin/zsh for example, it will not work cause the configure is not able to run on zsh.
I suppose that changing the default /bin/sh to something not sh compatible would break many other things.

 package/grep/grep.mk | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index be3e53b..7365a57 100644
@@ -10,7 +10,7 @@ GREP_SOURCE = grep-$(GREP_VERSION).tar.xz
 GREP_LICENSE = GPL-3.0+
 GREP_LICENSE_FILES = COPYING
 GREP_DEPENDENCIES = $(TARGET_NLS_DEPENDENCIES)
-GREP_CONF_OPTS = --exec-prefix=/
+GREP_CONF_OPTS = SHELL=/bin/sh --exec-prefix=/
 
 # link with iconv if enabled
 ifeq ($(BR2_PACKAGE_LIBICONV),y)

2.7.4