build: Move the extension definition to common place

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

boards/x86/qemu/qemu-i486/scripts/Make.defs

@@ -93,9 +93,6 @@ CFLAGS = $(ARCHWARNINGS) $(ARCHOPTIMIZATION) $(ARCHCPUFLAGS) $(ARCHINCLUDES) $(A
 CPPFLAGS = $(ARCHINCLUDES) $(ARCHDEFINES) $(EXTRAFLAGS)
 AFLAGS = $(CFLAGS) -D__ASSEMBLY__
 
-ASMEXT = .S
-OBJEXT = .o
-LIBEXT = .a
 EXEEXT = .elf
 
 ifeq ($(CONFIG_DEBUG_SYMBOLS),y)
@@ -105,4 +102,3 @@ endif
 HOSTCC = gcc
 HOSTINCLUDESv = -I.
 HOSTCFLAGS = -Wall -Wstrict-prototypes -Wshadow -Wundef -g -pipe
-HOSTLDFLAGS =