Use -Os for CC2538 builds

As discussed in #503, -Os was broken with one of the toolchains
recommended in the platform's README and for that reason we were
using -O2 by default.

This commit sets the default to -Os and updates the README to no
longer recommend the toolchain in question
Make some CC2538 CFLAGS and LDFLAGS common

Some CFLAGS and LDFLAGS previously only enabled with SMALL=1 have
now been enabled for all builds, regardless of the value of SMALL.

Therefore, from now on, SMALL only chooses between -Os and -O2