uboot: simplify patch wildcard for BR2_TARGET_UBOOT_CUSTOM_PATCH_DIR

To match the logic we have for BR2_LINUX_KERNEL_PATCH. The user has
already specified a custom patch directory, so we don't need to be
so specific about the what file names we accept, and it becomes quite
cumbersome when a custom git version is used.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>