Gluon: introduce build flag GLUON_LEGACY

[rotanid]

flag a target as legacy, requiring firmware builders to decide whether to build only sysupgrade or sysupgrade and factory images

GLUON_LEGACY={false,true,sysupgrade}

[NeoRaider]

While I don't have a strong opinion on this (and will hopefully not turn this issue into a bikeshedding discussion), I think I'll go with the following values for GLUON_LEGACY:

• 0: don't build any legacy images
• 1: build legacy sysupgrade images
• full: build legacy sysupgrade and factory images

This has two advantages in my opinion:

• 0/1 is already used for other options
• having a separate value for the "sysupgrade+factory" setting rather than for the "sysupgrade only" setting makes this option special, hopefully hinting site builders at not choosing this option ;)