Fix cmake configure for STM32F4XX and STM32F7XX (#59)

Fixed bug that prevented STM32F4XX and STM32F7XX from correctly
configure with cmake.

Description
-----------
Setting `FREERTOS_PLUS_FAT_PORT` to STM32F4XX would throw fatal error at
line 57 of root CMakeLists.txt since not in
`FREERTOS_PLUS_FAT_PORT_LIST`.
Setting `FREERTOS_PLUS_FAT_PORT` to STM32FXX as in the list of ports
(notice no number before XX), would throw error since at
`portable/CMakeLists.txt:38` it would NOT mach with any generator and
result in no sources for `freertos_plus_fat_port` target.

Test Steps
-----------
<!-- Describe the steps to reproduce. -->
set `FREERTOS_PLUS_FAT_PORT` to either STM32F4XX or STM32F7XX

Checklist:
----------
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that apply. -->
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here to help! -->
- [ ] I have tested my changes. No regression in existing tests.
- [ ] I have modified and/or added unit-tests to cover the code changes
in this Pull Request.

Related Issue
-----------
<!-- If any, please provide issue ID. -->

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CMakeLists.txt

@@ -36,7 +36,8 @@ set(FREERTOS_PLUS_FAT_PORT_LIST
     AVR32_UC3
     LPC18XX
     POSIX
-    STM32FXX
+    STM32F4XX
+    STM32F7XX
     STM32HXX
     ZYNQ
     ZYNQ_2019_3