build: allow DESTDIR setting

Allow Makefile to understand the DESTDIR setting.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
build: also install dirs before install files

Lets create dirs before actually installing files, so we can work
in environments where no previous dirs exist.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
build: fix conf file permissions at install stage

Install config file with 0644 instead of the `install' default 0755.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>