kbuild: Customflags for cmd_objcopy

From Konrad Eisele:

> like with cmd_ld in scripts/Makefile.lib having possibility to add 
> customflags with cmd_objcopy would be nice. When building a
> ROMKernel I'd like to use:
> OBJCOPYFLAGS_rompiggydata := --remove-section=.text
> OBJCOPYFLAGS_$(MODEL)piggytext := --only-section=.text

scripts/Makefile.lib

@@ -165,7 +165,7 @@ cmd_ld = $(LD) $(LDFLAGS) $(EXTRA_LDFLAGS) $(LDFLAGS_$(@F)) \
 # ---------------------------------------------------------------------------
 
 quiet_cmd_objcopy = OBJCOPY $@
-cmd_objcopy = $(OBJCOPY) $(OBJCOPYFLAGS) $< $@
+cmd_objcopy = $(OBJCOPY) $(OBJCOPYFLAGS) $(OBJCOPYFLAGS_$(@F)) $< $@
 
 # Gzip
 # ---------------------------------------------------------------------------