netsocket: dns: make dns-cache-size:0 remove whole DNS cache code

[TeroJaasko]

Description

Setting ""nsapi.dns-cache-size": 0" still left some of the DNS
caching code in. Add crude #if to remove all of it.
This allows one to save 429 bytes of flash and 48 bytes of RAM on
ARMC5 builds.

Pull request type

[x] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[mikaleppanen]

This is modified also in: #7207

This pull request can be used in case application does not want to use the cache at all. That was not possible on previous lwip implementation (unless changing not exposed lwip internal configuration settings), but is good feature for new DNS.

[SeppoTakalo]

Using a SingletonPointer would have prevented that. It does lazy construction, so if the object is not referenced, it's constructor would not be referenced either, allowing linker to drop it.

[kjbracey]

This will need to be rebased on that other PR #7207, but seems fine to me.

[kjbracey]

I don't think SingletonPtr totally solves this case - it would eliminate the static cache structures not referenced due to the #if > 0 check, but probably generate an "unreferenced static" warning. So the combined PRs should have the SingletonPtrs surrounded by the #if > 0

[TeroJaasko]

Actually the original code does not even compile on IAR with ""nsapi.dns-cache-size": 0", so some kind of conditional compilation is needed:

[Error] nsapi_dns.cpp@113,0: [Pe094]: the size of an array must be greater than zero
[ERROR] 
  static DNS_CACHE *dns_cache[MBED_CONF_NSAPI_DNS_CACHE_SIZE];

[0xc0170]

This will need to be rebased on that other PR #7207, but seems fine to me.

Can you rebase? There is a conflict now

[cmonr]

@TeroJaasko Would you mind rebasing?

[TeroJaasko]

Rebased with current master and force pushed.

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2487
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7239/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2126
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7239/

[mbed-ci]

Test : SUCCESS

Build number : 2262
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7239/2262