[make] extend the top level make file to set the root of lk and pass toolchain_prefix via make variable
[make] extend the top level make file to set the root of lk and pass toolchain_prefix via make variable

littlekernel/lk,travisg/lk,travisg/lk,skabet/lk,grub4android/lk,M1cha/lk,sndnvaps/lk,srodrig1/lk,GCrean/lk,srodrig1/lk,GCrean/lk,srodrig1/lk,nvll/lk,M1cha/lk,littlekernel/lk,grub4android/lk,sndnvaps/lk,hollanderic/lkstuff,nvll/lk,grub4android/lk,grub4android/lk,ErikCorryGoogle/lk,grub4android/lk,hollanderic/lkstuff,sndnvaps/lk,hollanderic/lkstuff,srodrig1/lk,littlekernel/lk,M1cha/lk,skabet/lk,skabet/lk,nvll/lk,M1cha/lk,minglun-tsai/lk,skabet/lk,sndnvaps/lk,travisg/lk,minglun-tsai/lk,nvll/lk,nvll/lk,travisg/lk,minglun-tsai/lk,hollanderic/lkstuff,GCrean/lk,GCrean/lk,sndnvaps/lk,ErikCorryGoogle/lk,minglun-tsai/lk,littlekernel/lk,srodrig1/lk,ErikCorryGoogle/lk,GCrean/lk,ErikCorryGoogle/lk,skabet/lk,M1cha/lk