build: fix libjpeg-turbo build for armv6
build: remove arm flags for emulator
Merge pull request #247 from koreader/kindle-legacy

build: fix libjpeg-turbo build for armv6
implement a "viewport" blitbuffer for another blitbuffer

This allows to create a new blitbuffer that presents a rectangular
area of another blitbuffer: BBnew(x,y) = BB(offset_x+x,offset_y+y)