Revert "libubox: use PKG_ABI_VERSION"

Revert "libubox: use PKG_ABI_VERSION"

This fixes the build on MIPS BE like ath25 and ath79 target.
We get this error message when linking libubox:
mips-openwrt-linux-musl/bin/ld: /home/hauke/openwrt/openwrt/staging_dir/target-mips_mips32_musl/usr/lib/libubox.so: unknown type [0x7000002a] section `.MIPS.abiflags'
mips-openwrt-linux-musl/bin/ld: /home/hauke/openwrt/openwrt/staging_dir/target-mips_mips32_musl/usr/lib/libubox.so: unknown type [0x7000002a] section `.MIPS.abiflags'
mips-openwrt-linux-musl/bin/ld: skipping incompatible /home/hauke/openwrt/openwrt/staging_dir/target-mips_mips32_musl/usr/lib/libubox.so when searching for -lubox

This reverts commit f421fefa8a34319c5ff5dcc1af39d6311ec1ad1e.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>