pkg_alternatives: pass if the desired symlink already exists

pkg_alternatives: pass if the desired symlink already exists

This can happen when opkg installs, then configures multiple
alternatives in a batch.  Symlink to the highest prio alternative will
be created in the initial configuring stage causing later symlink call
fail with EEXIST

    Configuring busybox.
    ...
    symlink("/sbin/ip-full", "/home/yousong/j/t/lede-imagebuilder-mvebu.Linux-x86_64/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/root-mvebu/sbin/ip") = 0
    ...
    Configuring ip-full.
    ...
    symlink("/sbin/ip-full", "/home/yousong/j/t/lede-imagebuilder-mvebu.Linux-x86_64/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/root-mvebu/sbin/ip") = -1 EEXIST (File exists)
    ...

While at it, "mkdir -p" dirname(path_in_dest) before symlink in case the
following symlink call may fail with ENOENT

Ref: https://github.com/openwrt/packages/issues/4567
Reported-by: Aner Andros <aa@anerandros.info>
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>