mips: vdso: Fix flip/flop vdso building bug

mips: vdso: Fix flip/flop vdso building bug

Running "make" on an already compiled kernel tree will rebuild the
vdso library even if this has not been modified.

$ make
  GEN     Makefile
  Using linux as source for kernel
  CALL   linux/scripts/atomic/check-atomics.sh
  CALL   linux/scripts/checksyscalls.sh
<stdin>:1511:2: warning: #warning syscall clone3 not implemented [-Wcpp]
  CHK     include/generated/compile.h
  VDSO    arch/mips/vdso/vdso.so.dbg.raw
  OBJCOPY arch/mips/vdso/vdso.so.raw
  GENVDSO arch/mips/vdso/vdso-image.c
  CC      arch/mips/vdso/vdso-image.o
  AR      arch/mips/vdso/built-in.a
  AR      arch/mips/built-in.a
  CHK     include/generated/autoksyms.h
  GEN     .version
  CHK     include/generated/compile.h
  UPD     include/generated/compile.h
  CC      init/version.o
  AR      init/built-in.a
  LD      vmlinux.o
  MODPOST vmlinux.o
  MODINFO modules.builtin.modinfo
  KSYM    .tmp_kallsyms1.o
  KSYM    .tmp_kallsyms2.o
  LD      vmlinux
  SORTEX  vmlinux
  SYSMAP  System.map
  Building modules, stage 2.
  ITS     arch/mips/boot/vmlinux.gz.its
  OBJCOPY arch/mips/boot/vmlinux.bin
  MODPOST 7 modules
  GZIP    arch/mips/boot/vmlinux.bin.gz
  ITB     arch/mips/boot/vmlinux.gz.itb

The issue is generated by the fact that "if_changed" is called twice
in a single target.

Fix the build bug merging the two commands into a single function.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>