bpf: ignore missing LLVM bins on package for non compile steps

bpf: ignore missing LLVM bins on package for non compile steps

To download a package the LLVM bins are not strictly needed.
Currently with an example run of make package/bridger/download V=s, the
build fail with

make[2]: Entering directory '/home/ansuel/openwrt-ansuel/openwrt/package/network/services/bridger'
bash: line 1: /home/ansuel/openwrt-ansuel/openwrt/staging_dir/host/llvm-bpf/bin/clang: No such file or directory
bash: line 1: [: : integer expression expected
/home/ansuel/openwrt-ansuel/openwrt/include/bpf.mk:71: *** ERROR: LLVM/clang version too old. Minimum required: 12, found: .  Stop.
make[2]: Leaving directory '/home/ansuel/openwrt-ansuel/openwrt/package/network/services/bridger'
time: package/network/services/bridger/download#0.04#0.00#0.06
    ERROR: package/network/services/bridger failed to build.

This is wrong since it may be needed to download the required packages
first and then compile them later.

Fix this by ignoring the LLVM bin check on non compile steps.

Tested-by: Robert Marko <robimarko@gmail.com>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>