Allow CFLAGS/CPPFLAGS to be completed from the environment

Buildroot passes its own CPPFLAGS and CFLAGS in the environment, so
the CFLAGS += and CPPFLAGS += statements in gnu-efi Makefile have no
effect. Change these to override <VARIABLE> += so that they extend the
flags passed by Buildroot.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/Make.defaults
===================================================================
--- a/Make.defaults
+++ b/Make.defaults
@@ -83,25 +83,25 @@
                                && [ $(GCCMINOR) -ge "7" ] ) ) \
                         && echo 1)
 ifeq ($(GCCNEWENOUGH),1)
-  CPPFLAGS += -DGNU_EFI_USE_MS_ABI -maccumulate-outgoing-args --std=c11
+  override CPPFLAGS += -DGNU_EFI_USE_MS_ABI -maccumulate-outgoing-args --std=c11
 endif
 
 # Arch-specific compilation flags
-CPPFLAGS += -DCONFIG_$(ARCH)
+override CPPFLAGS += -DCONFIG_$(ARCH)
 
 ifeq ($(ARCH),ia64)
-  CFLAGS += -mfixed-range=f32-f127
+  override CFLAGS += -mfixed-range=f32-f127
 endif
 
 ifeq ($(ARCH),ia32)
-  CFLAGS += -mno-mmx -mno-sse
+  override CFLAGS += -mno-mmx -mno-sse
   ifeq ($(HOSTARCH),x86_64)
     ARCH3264 = -m32
   endif
 endif
 
 ifeq ($(ARCH),x86_64)
-  CFLAGS += -mno-red-zone -mno-mmx -mno-sse
+  override CFLAGS += -mno-red-zone -mno-mmx -mno-sse
   ifeq ($(HOSTARCH),ia32)
     ARCH3264 = -m64
   endif
@@ -110,7 +110,7 @@
 # Generic compilation flags
 INCDIR  += -I$(SRCDIR) -I$(TOPDIR)/inc -I$(TOPDIR)/inc/$(ARCH) \
            -I$(TOPDIR)/inc/protocol
-CFLAGS  += $(ARCH3264) -O2 -fpic -Wall -fshort-wchar -fno-strict-aliasing \
+override CFLAGS  += $(ARCH3264) -O2 -fpic -Wall -fshort-wchar -fno-strict-aliasing \
            -fno-merge-constants -ffreestanding -fno-stack-protector       \
            -fno-stack-check
 ASFLAGS += $(ARCH3264)