At present, when running a busybox shell ("busybox sh") from mksh
(/system/bin/sh), one gets the following message before every command line:
sh: precmd: Permission denied
This is due to mkshrc exporting a PS1 which depends on a mkshrc defined
function, "precmd". Since "busybox sh" does not source mkshrc, it does not
have this function defined, resulting in the above error.
By not exporting PS1, "busybox sh" uses its own reasonable default. mksh
subshells continue to work as usual, since they (re)source /etc/mkshrc
anyways.
Change-Id: Ic9182e42aa033e811300d9e6e8d6d3f43e14e58f
