Support for SPL framework on STM32L1

[ivankravets]

Moved from platformio/platformio-core#348

/cc @laurentS

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I'm testing code on ST Nucleo L152RE, which is based on STM32L152 mcu. I found code examples that use SPL framework for this MCU, but cannot get them to compile with platformio.
When I try to add SPL to frameworks (along with mbed), I get a "SPL not supported for this platform" error.
Why is this? I understand that SPL is linked to the MCU, not the board. So I'm expecting I should be able to use it for that MCU. Any suggestions?

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