Standard open mode users (i.e. the kernel is not patched to neuter aegis) will need to install aegisctl, since aegis will still protect entries in /sys/kernel/security/validator/.

So the correct procedure for standard open mode users would be:
1. Install aegisctl
2. AEGIS_FIXED_ORIGIN=com.nokia.maemo dpkg -i busybox-power_1.21.0power1-harmattan0_armel.deb

Only if you're running a kernel that does neuter aegis (i.e. Patched open mode), you can leave out step 1 and use the -noaegis version.

By the way, if you were to install the -noaegis version on a non-neutered system, the postinst script would simply error out (busybox-power will then be left unconfigured for the moment) so your system won't be broken. All (invalid) scenario's are handled correctly by the packaging .


Edit:
You can't write to /sys/kernel/security/validator/* if you aren't running a kernel that neuters aegis, even when you're in open mode. So you'll still need aegisctl to unseal aegis.