u-boot-mfgtool.inc: Align SPL_IMAGE and SPL_SYMLINK with OE
The deploy task of u-boot-imx-mfgtool fails due to the incorrect
addition of an 'spl' sub-folder:
```
install: cannot create regular file '.../tmp/work/imx8mq_evk-fsl-linux/u-boot-imx-mfgtool/2021.04-r0/deploy-u-boot-imx-mfgtool/spl/u-boot-spl.bin-imx8mq-evk-mfgtool-2021.04-r0-mfgtool-2021.04-r0': No such file or directory
```
The customization of SPL_IMAGE and SPL_SYMLINK for mfgtool should
just add -mfgtool to the values defined in OE-core. Instead, the
base SPL_BINARYNAME is changed to SPL_BINARY, which is why the
extra folder 'spl' is in the path.
Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>
