build: copy contents of 'src' folder to build dirs (if present)

build: copy contents of 'src' folder to build dirs (if present)

The normal Prepare step for a build is unpack, apply patches.
But for certain packages, patches contain whole files, which
would be nice to have separately and copied over as a last step
in the Prepare phase.

We need it for some other packages + patches, but I think
the 'hostapd' package can be used as a test for this.

As a quick note:
the reason the condition is being evaluated as
`[ ! -d ./src/ ] || $(CP) ./src/* $(HOST_BUILD_DIR)`
and not with
`[ -d ./src/ ] && $(CP) ./src/* $(HOST_BUILD_DIR)`
is that the latter would translate in a build failure if the `src`
folder is not present (the exit code would be 1).
The first one, succeeds for both cases (if `src` present or not).

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>