mp3_encoder: reduce plugin binsize
- reduce the size of some structs -> gain ~1kB
- since we run the code only once, we don't need to declare large
structure in bss and copy const data in them, declare them const
directly -> gain ~17kB
- steal audio buffer for encoded data buffer -> gain 64kB
this will stop playback when encoding, but it was stopped already on
targets using IRAM
- some const policy
Tested on Clipv1 & sim, 83012 bytes smaller on clipv1
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@26134 a1c6a512-1295-4272-9138-f99709370657