Makefile: fix broken pipe error for lcd4_lwmon5 board

Before this commit, a broken pipe error sometimes happened
when building lcd4_lwmon5 board with Buildman.

This commit re-writes build rules of
u-boot.spr and u-boot-img-spl-at-end.bin
more simply without using a pipe.

Besides fixing a broken pipe error,
this commit gives us other advantages:

  - Do not generate intermidiate files, spl/u-boot-spl.img
    and spl/u-boot-spl-pad.img for creating u-boot.spr

  - Do not generate an intermidiate file, u-boot-pad.img
    for creating u-boot-img-spl-at-end.bin

Such intermidiate files were not deleted by "make clean" or "make mrpropr".
Nor u-boot-pad.img was ignored by git.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Stefan Roese <sr@denx.de>