config file is what you need. I figured it out.
You need to have a .ebextensions/ in the top level of your app. Then place a myapp.config in the ebtextensions directory.
here is what i have that installs the packages
packages:
yum:
ImageMagick : ""
bison : ""
byacc : ""
cscope: ""
ctags: ""
cvs: ""
diffstat: ""
doxygen: ""
flex: ""
gcc: ""
gcc-c++: ""
gcc-gfortran: ""
gettext: ""
git: ""
indent: ""
intltool: ""
libtool: ""
patch: ""
patchutils: ""
rcs: ""
rpm-build: ""
subversion: ""
swig: ""
systemtap: ""
openssl-devel: ""
ImageMagick-c++: ""
ImageMagick-c++-devel: ""
crash: ""
kexec-tools: ""
latrace: ""
ltrace: ""
strace: ""
system-rpm-config: ""
texinfo: ""
valgrind: ""
busybox: ""