[MACHINE][IMX8MP-EVK][FIX] fix IMX_DEFAULT_BOOTLOADER

By default, IMX_DEFAULT_BOOTLOADER is set to "u-boot-fslc", and as a result
uuu_bootloader_tag expects the flash.bin file in the deploy directory (instead
of imx-boot), causing this error at build time:

  cp: cannot stat '/home/yoctouser/workdir/build-imx8mpevk/tmp/work/nxp_imx8mpevk-poky-linux/imx-boot/1.0/deploy-imx-boot/flash.bin': No such file or directory

Set it to "u-boot-imx" to fix that.

Signed-off-by: Alessandro Pecugi <alessandro.pecugi@seco.com>

conf/machine/nxp-imx8mpevk.conf

@@ -22,6 +22,8 @@ SPL_BINARY = "spl/u-boot-spl.bin"
 IMX_BOOT_SOC_TARGET = "iMX8MP"
 IMXBOOT_TARGETS = "flash_evk"
 
+IMX_DEFAULT_BOOTLOADER = "u-boot-imx"
+
 ATF_PLATFORM = "imx8mp"
 
 OPTEE_BIN_EXT = "8mp"