grep: fix that egrep and fgrep workaround doesn't work

Commit 07b6eec21f doesn't work at least
now, because package.mk initializes the variables to the default
values.  You have to modify the variable after including package.mk.

Signed-off-by: Kazuhiro Ito <kzhr@d1.dion.ne.jp>
(cherry picked from commit b635f8ab9a)

utils/grep/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=grep
 PKG_VERSION:=3.11
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.xz
 PKG_SOURCE_URL:=@GNU/grep
@@ -23,10 +23,10 @@ PKG_CPE_ID:=cpe:/a:gnu:grep
 PKG_INSTALL:=1
 PKG_BUILD_PARALLEL:=1
 
-MAKE_FLAGS += SHELL="/bin/sh"
-
 include $(INCLUDE_DIR)/package.mk
 
+MAKE_FLAGS += SHELL="/bin/sh"
+
 define Package/grep
   SECTION:=utils
   CATEGORY:=Utilities