imx8qxp: Fix build when using SPL

imx8qxp: Fix build when using SPL

imx8qxp based boards which use SPL drop error when
calling make all:
"""
Writing image to './flash.bin'
   Node '/binman/imx-boot/spl': GetData: size 0x0
       Node '/binman/imx-boot': GetPaddedDataForEntry: size 0x0
       Node '/binman/imx-boot': GetData: 1 entries, total size 0x0
       Node '/binman/imx-boot': GetPaddedDataForEntry: size 0x0
Wrote 0x0 bytes
Image 'imx-boot' is missing external blobs and is non-functional: spl

/binman/imx-boot/spl (spl.bin):
   Missing blob

Some images are invalid
"""

Guard creation of flash.bin with CONFIG_XPL_BUILD option.

Signed-off-by: Heiko Schocher <hs@denx.de>
Fixes: c9713c155127 ("imx8-u-boot: Fix SPL guard option")