openssl: rework CFLAGS handling on compilation

openssl: rework CFLAGS handling on compilation

This mainly improve the CFLAGS handling on compilation of OpenSSL.

The CFLAGS are currently passed 2 times generating compilation warning
due to -fhonour-copts passed 2 times.

This can be improved by passing the CFLAGS as env to the OpenSSL
Configure tool.

For consistency we do the same for CPPFLAGS and LDFLAGS.

This permits to drop redundant flags in the Compile phase and from the
.conf file.

Link: https://github.com/openwrt/openwrt/pull/20665
Signed-off-by: Christian Marangi <[email protected]>