net/chan-sccp-b: version bump and fixes

net/chan-sccp-b: version bump and fixes

- Bump to revision 6728.
- Remove '-fi' from PKG_FIXUP. Trust in the build system to do the right
  thing.
- Set PKG_INSTALL:=1 so "make install" doesn't need to be called
  manually anymore. This also drops LOW_MEMORY from CFLAGS, which is not
  needed as chan-sccp-b looks into asterisk's headers to find out if
  LOW_MEMORY is enabled or not.
  The build output actually looks much more pleasant afterward.
- Disable chan-sccp-b's optimization which it enables by default. It
  sets -O3 and all that jazz. To make it simply use the OpenWrt/LEDE
  flags optimization needs to be disabled.
- With optimization disabled chan-sccp-b still adds -Og to the end of
  the CFLAGS. Add a small patch to prevent that.
- Disable debug which is enabled by default. This brings down the size
  of the installed binary from 14 MByte (!) to a little over 2 MByte
  (checked on ARM).

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>