bootman: only do partitionless boot when the bootloader supports it

[silkeh]

#174 restored support for partitionless /boot. However, this introduced issues when (for whatever reason) there is content in the /boot directory (instead of being empty) (see #175).

As partitionless boot is not an option for every bootloader, add a flag indication bootloader support. Bootloaders without partitionless support (and those on UEFI) will never try to use a partitionless /boot.