common/libexec: remove redundant pre/do/post preparation code

This pullrequest removes redundant codepaths in
xbps-src-do{build,configure,install}.sh and joins the code in the
run_step function. This causes slightly different behavior to
do_install:
Do install will chdir to wrksrc only _before_ the first step. The
current behavior is that pre_install will run without a chdir, do_ and
post_ is runned with a chdir. This is a subtle but breaking change and
may cause some templates to break at install phase.

common/xbps-src/libexec/xbps-src-dobuild.sh

@@ -31,41 +31,7 @@ for f in $XBPS_COMMONDIR/environment/build/*.sh; do
     source_file "$f"
 done
 
-cd "$wrksrc" || msg_error "$pkgver: cannot access wrksrc directory [$wrksrc]\n"
-if [ -n "$build_wrksrc" ]; then
-    cd $build_wrksrc || \
-        msg_error "$pkgver: cannot access build_wrksrc directory [$build_wrksrc]\n"
-fi
-
-run_pkg_hooks pre-build
-
-# Run pre_build()
-if declare -f pre_build >/dev/null; then
-    run_func pre_build
-fi
-
-# Run do_build()
-if declare -f do_build >/dev/null; then
-    run_func do_build
-else
-    if [ -n "$build_style" ]; then
-        if [ ! -r $XBPS_BUILDSTYLEDIR/${build_style}.sh ]; then
-            msg_error "$pkgver: cannot find build helper $XBPS_BUILDSTYLEDIR/${build_style}.sh!\n"
-        fi
-        . $XBPS_BUILDSTYLEDIR/${build_style}.sh
-        if declare -f do_build >/dev/null; then
-            run_func do_build
-        fi
-    fi
-fi
-
-
-# Run post_build()
-if declare -f post_build >/dev/null; then
-    run_func post_build
-fi
-
-run_pkg_hooks post-build
+run_step build
 
 touch -f $XBPS_BUILD_DONE