Fix {build.mcu} definition

Library can provide a precompiled library.
See precompiled option here:
https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5:-Library-specification#libraryproperties-file-format

The .a(archive)/.so(shared object) file must be located at: `src/{build.mcu}`

So the `build.mcu` could not contain `fpu` and `float-abi` options
and should only contain `cortex-mx` value.

Else Arduino IDE, instead of searching `cortex-mx`, searches
`cortex-mx -mfpu=fpv4-sp-d16 -mfloat-abi=hard` if floating-point
options are set.

`build.flags.fp` has been added to contain floating-point options if any.

Fixes #606

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Author: fpistm