Do not hardcode warning and optimisation flags, otherwise the
main options (i.e. DBG, CPPFLAGS) will not work as expected.
You can still provide specific default by using DBG?=<value>.
Doing so leaves the opportunity to override the setting from the
commandline, while the default value from the build system is
then ignored for that particular package.
When crosscompiling, and using build.sh, adding -V DBG=<value> has
this same effect as make DBG=<value>.
Change-Id: Ic610e4d33b945acad64571e1431f1814291e2d84
c7b7a6ea29