Generated SDK environment files use 'source' to load additional environment files. On some dash shells, this will cause an immediate failure. 'source' is not supported, but '.' is. ('source' is a csh-ism that bash picked up.. so it's not posix -- but '.' is.) The problem is most likely in the 'toolchain-scripts.bbclass' where it generated the environment files.
Assigning to Stephano to let him explore various bits of the codebase :)