The configs/omap3_overo_defconfig file was removed from upstream U-Boot
in commit ed3294d6d1f9 ("arm: Remove overo board"). Remove it in OpenWrt
too. If someone needs this please add it also to upstream U-Boot.
This fixes the compile of the omap target.
Fixes: ffb807ec90d3 ("omap: update u-boot to 2021.07")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>