When we have entered the chroot'ed environment in the next section we want to export a couple of environment variables in that shell such as TERM, CFLAGS, CXXFLAGS and others variables you want to have set. For that purpose we'll create the $LFS/root/.bash_profile file and "source" it after we have entered the chroot'ed environment.
Create a new file $LFS/root/.bash_profile by running the following. Replace -march=xxx with the proper value for your machine:
cat > $LFS/root/.bash_profile << "EOF"
# Begin /root/.bash_profile
alias ls='ls -a --color'
PS1='\u:\w\$ '
TERM=linux
CFLAGS="-O3 -march=xxx"
CXXFLAGS=$CFLAGS
export PS1 TERM CFLAGS CXXFLAGS
# End /root/.bash_profile
EOF
You can add more environment variables at your own discretion as you deem them necesarry.