This avoids a ton of duplication and makes it easier to change a generic
target (for example, the "cortex-m" target) for all boards that use it.
Also, by making it possible to inherit properties from a parent target
specification, it is easier to support out-of-tree boards that don't
have to be updated so often. A target specification for a
special-purpose board can simply inherit the specification of a
supported chip and override the properites it needs to override (like
the programming interface).
Be more compatible with the Go toolchain by setting GOPATH in the same
way. This makes it possible to flash and run examples from the standard
GOPATH instead of only from the source tree.
This fixes a problem on baremetal targets, where PIE doesn't make any
sense. Specifically, on ARM, the compiler sometimes inserted GOT
pointers for linker-defined globals.
