uboot-envtools: move imx to imx_cortexa9

Subtarget-specific files under 'uboot-envtools' package are supported
since 6f3a05ebb0 ("uboot-envtools: support uci-default config also per
subtargets").

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>

diff --git a/package/boot/uboot-envtools/files/imx b/package/boot/uboot-envtools/files/imx_cortexa9
similarity index 100%
rename from package/boot/uboot-envtools/files/imx
rename to package/boot/uboot-envtools/files/imx_cortexa9