I had similar problems and have been able to update. You should update 
sysvinit first with pacman -S sysvinit and the command given by kmihelich will do the trick.
History of commands. Not all of them should be used but it worked for me so I let them here 
 
 $this->bbcode_second_pass_code('', 'pacman -S filesystem
pacman -S kmod
pacman -S linux-headers
pacman -S sysvinit
pacman -S linux
pacman -Syu
rm /etc/profile.d/locale.sh
pacman -Syu')
Initial situation$this->bbcode_second_pass_code('', '
[root@alarm ~]# pacman -Syu
:: Synchronizing package databases...
 core                                                       184.0   B  12.0K/s 00:00 [################################################] 100%
 core is up to date
 extra                                                      184.0   B  1321K/s 00:00 [################################################] 100%
 extra is up to date
 community                                                  184.0   B   478K/s 00:00 [################################################] 100%
 community is up to date
 alarm                                                      184.0   B   488K/s 00:00 [################################################] 100%
 alarm is up to date
 aur                                                        184.0   B   475K/s 00:00 [################################################] 100%
 aur is up to date
:: Starting full system upgrade...
:: Replace kernel26 with core/linux? [Y/n] 
:: Replace kernel26 with core/linux-mmp? [Y/n] 
:: Replace kernel26 with core/linux-raspberrypi? [Y/n] 
:: Replace kernel26-headers with core/linux-headers? [Y/n] 
:: Replace kernel26-headers with core/linux-headers-mmp? [Y/n] 
:: Replace kernel26-headers with core/linux-headers-raspberrypi? [Y/n] 
:: Replace module-init-tools with core/kmod? [Y/n] 
:: Replace procps with core/procps-ng? [Y/n] 
resolving dependencies...
looking for inter-conflicts...
warning: removing 'linux' from target list because it conflicts with 'linux-mmp'
error: unresolvable package conflicts detected
error: failed to prepare transaction (conflicting dependencies)
:: linux-raspberrypi and linux-mmp are in conflict')