ARM: Parse command line delivered by U-Boot:

• in atags
• in DT blob (by using 'fdt chosen' U-Boot command)

The command line must start with guard's string 'FreeBSD:' and can contain
list of comma separated kenv strings. Also, boot modifier strings from
boot.h are recognised and parsed into boothowto.

The command line must be passed from U-Boot by setting of bootargs variable:
'setenv bootargs FreeBSD:boot_single=1,vfs.root.mountfrom=ufs:/dev/ada0s1a'
followed by 'fdt chosen' (only for DT based boot)