Three changes are needed to accommodate xc32 compiler build:
- optimized build flag other than -Os
added CFLAGS_OPTIMIZED that defaults to -Os but can be overridden in boards
- build without -lnosys
added LIBS_GCC with default libraries that can be changed in boards
- build without LD_FILE specification
if LD_FILE is empty -Wl,-T options is not added to LDFLAGS
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