Hint: jam clean for hybrid build needs to be executed in both folders

[rossi]

The following page should be amended with a little warning hint to remind people of some catches when performing hybrid builds:

​http://www.haiku-os.org/guides/building/gcc-hybrid

For hybrid builds jam clean needs to be invoked in both the primary and secondary build folder, as jam clean apperently doesn't handle this situation automatically.

A small hint on the hybrid building site mentioning this fact would be nice ;-) as it prevents one from assuiming that jam clean cleans everything, as when building the sub jam is also automatically invoked. For cleaning this doesn't apply.

[pulkomandy]

Rework of the buildsystem for PM support fixed this.