build: Move the extension definition to common place

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

boards/arm/str71x/olimex-strp711/scripts/Make.defs

@@ -112,11 +112,6 @@ NXFLATLDFLAGS2 = $(NXFLATLDFLAGS1) \
                  -no-check-sections
 LDNXFLATFLAGS = -e main -s 2048
 
-ASMEXT = .S
-OBJEXT = .o
-LIBEXT = .a
-EXEEXT =
-
 ifneq ($(CROSSDEV),arm-nuttx-elf-)
   LDFLAGS += -nostartfiles -nodefaultlibs
 endif
@@ -127,4 +122,3 @@ endif
 HOSTCC = gcc
 HOSTINCLUDES = -I.
 HOSTCFLAGS = -Wall -Wstrict-prototypes -Wshadow -Wundef -g -pipe
-HOSTLDFLAGS =