Fix OpenSSL cross-compaile warnings

[noloader]

This PR cleans up OpenSSL warnings when cross-compiling. It also uses the same 15-android.conf and setenv-android.sh as Unbound.

OpenSSL has a penchant for ignoring user flags. When Travis jobs were added that used ANDROID_API=23 (the minimum) it produced warnings like the following because OpenSSL stomped on Unbound's flags.

armv7a-linux-androideabi23-clang  -I. -Iinclude -fPIC -pthread -D__ANDROID_API__
=23 -march=armv7-a -mthumb -mfloat-abi=softfp -funwind-tables -fexceptions --sys
root=/home/travis/android-ndk/toolchains/llvm/prebuilt/linux-x86_64/sysroot -DOP
ENSSL_USE_NODELETE -DOPENSSL_PIC -DOPENSSLDIR="\"/home/travis/android23-armv7a\"
" -DENGINESDIR="\"/home/travis/android23-armv7a/lib/engines-1.1\"" -D__ANDROID_A
PI__=29 -DNDEBUG  -MMD -MF apps/app_rand.d.tmp -MT apps/app_rand.o -c -o apps/ap
p_rand.o apps/app_rand.c

In file included from <built-in>:374:
<command line>:6:9: warning: '__ANDROID_API__' macro redefined
      [-Wmacro-redefined]
#define __ANDROID_API__ 29
        ^
<command line>:1:9: note: previous definition is here
#define __ANDROID_API__ 23
        ^

The issue is cleared by removing the function android_ndk from 15-android.conf.

A sample build with the noisy warnings is available here

[gthess]

Was it supposed to be arm64* here or the change is indeed intended?
The arm64-v8a option is not accounted for anymore but I also see that we don't use it with travis.

[noloader]

I believe Android uses arm64 alone. But arm64* should be fine too. arm64* will help users who do something like ANDROID_CPU=arm64-v8a. I'd make the change if I were you.

[gthess]

👍

[noloader]

@gthess, I grabbed the change for Crypto++ at Commit ccb3e869eddd. Thanks.