arm: mx5: Fix NAND image generation

The echo -ne "\xNN" does not work in certain bourne-compatible shells, like
dash. The recommended way of hex->char conversion is using printf(1), but
there is a pitfall here. The GNU printf does support "\xNN" format, but
according to the opengroup documentation, this is not part of POSIX. The
POSIX printf only defines "\NNN" where N is octal. Thus, for the sake of
compatibility, we use that.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>

arch/arm/imx-common/Makefile

@@ -86,7 +86,7 @@ u-boot-with-nand-spl.imx: spl/u-boot-nand-spl.imx u-boot.uim FORCE
 	$(call if_changed,pad_cat)
 
 quiet_cmd_u-boot-nand-spl_imx = GEN     $@
-cmd_u-boot-nand-spl_imx = (echo -ne '\x00\x00\x00\x00\x46\x43\x42\x20\x01' && \
+cmd_u-boot-nand-spl_imx = (printf '\000\000\000\000\106\103\102\040\001' && \
 	dd bs=1015 count=1 if=/dev/zero 2>/dev/null) | cat - $< > $@
 
 spl/u-boot-nand-spl.imx: SPL FORCE