fixup! pkg: enhance doxygen packages documentation

pkg/fatfs/doc.txt

 /**
  * @defgroup pkg_fatfs   FAT file system
  * @ingroup  pkg
- * @ingroup  sys
+ * @ingroup  sys_fs
  * @brief    Provides FAT file system support
  * @see      http://elm-chan.org/fsw/ff/00index_e.html
  */
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pkg/libfixmath/doc.txt

 /**
- * @defgroup pkg_lib_fix_math Cross platform fixed point maths library
+ * @defgroup pkg_libfixmath Cross platform fixed point maths library
  * @ingroup  pkg
  * @ingroup  sys
  * @brief    Provides a cross platform fixed point maths library to RIOT.

pkg/micro-ecc/doc.txt

 /**
  * @defgroup pkg_micro_ecc   Micro-ECC for RIOT
  * @ingroup  pkg
- * @ingroup  sys
+ * @ingroup  sys_crypto
  * @brief    Micro-ECC for RIOT
  *
  * # Micro-ECC for RIOT

pkg/relic/doc.txt

 /**
  * @defgroup pkg_relic  Relic toolkit for RIOT
  * @ingroup  pkg
- * @ingroup  sys
+ * @ingroup  sys_crypto
  * @brief    Provides the Relic cryptographic toolkit to RIOT
  *
  *

pkg/spiffs/doc.txt

 /**
  * @defgroup pkg_spiffs   SPI flash file system
  * @ingroup  pkg
- * @ingroup  sys
+ * @ingroup  sys_fs
  * @brief    Provides a file system for SPI NOR flash devices
  * @see      https://github.com/pellepl/spiffs
  */
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pkg/tiny-asn1/doc.txt

 /**
  * @defgroup pkg_tiny-asn1 Lightweight ASN.1 decoding/encoding library
  * @ingroup  pkg
- * @ingroup  sys
+ * @ingroup  sys_crypto
  * @brief    Lightweight ASN.1 decoding/encoding library
  * @see      https://gitlab.com/matthegap/tiny-asn1
  */

pkg/wakaama/doc.txt

 /**
  * @defgroup pkg_wakaama  Wakaama LwM2M implementation
  * @ingroup  pkg
+ * @ingroup  net
  * @brief    Provides the Wakaama implementation of LwM2M
  * @see      https://github.com/eclipse/wakaama
  */
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