retarget: fix microlib for mbed 2

[0xc0170]

only for mbed 2 needed.

This is as it used to be, reverting the removal. uARM does not have any hook
that we could use for mbed sdk init and copy nvic, therefore _open is used that
should be sufficient, but not ideal.

For more information, visit https://github.com/ARMmbed/mbed-os/pull/2160/files#r76563844.

cc @c1728p9 @bulislaw @jeromecoutant @LMESTM

[bulislaw]

LGTM

[jeromecoutant]

Hi
I am not able to review it, but first tests are OK now:

| OK | NUCLEO_L476RG | uARM | DTCT_1 | Simple detect test | 0.47 | 10 | 1/1 |
| OK | NUCLEO_L476RG | uARM | EXAMPLE_1 | /dev/null | 3.52 | 20 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_10 | Hello World | 0.39 | 5 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_11 | Ticker Int | 11.39 | 15 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_12 | C++ | 1.44 | 10 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_16 | RTC | 4.59 | 20 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_2 | stdio | 0.8 | 20 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_23 | Ticker Int us | 11.37 | 15 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_24 | Timeout Int us | 11.52 | 15 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_25 | Time us | 11.39 | 15 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_26 | Integer constant division | 1.42 | 20 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_34 | Ticker Two callbacks | 11.41 | 15 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_37 | Serial NC RX | 7.03 | 20 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_38 | Serial NC TX | 5.97 | 20 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_A1 | Basic | 1.35 | 20 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_A21 | Call function before main (mbed_main) | 1.45 | 20 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_A28 | CAN loopback test | 10.49 | 20 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_A30 | CAN API | 1.39 | 20 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_A9 | Serial Echo at 115200 | 1.54 | 20 | 1/1 |
| OK | NUCLEO_L476RG | uARM | MBED_BUSOUT | BusOut | 2.29 | 30 | 1/1 |

[LMESTM]

@0xc0170 LGTM
I had not noticed that this piece of code had disappeared ... is there a CI test with µARM boot ?

[0xc0170]

/morph test

[mbed-bot]

Result: FAILURE

Your command has finished executing! Here's what you wrote!

/morph test

Output

mbed Build Number: 705

Test failed!

[0xc0170]

/morph test

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph test

Output

mbed Build Number: 708

All builds and test passed!