examples: restore host builds on samd2x_l2x

Nine examples/host/*/only.txt gate on family:samd21, but 2a8811ebb merged the
samd2x and saml2x BSPs into hw/bsp/samd2x_l2x. skip_example takes `family:` from
the directory name, so since that rename every one of these examples has been
skipped on every board of the family, under make as well as cmake - although
hw/bsp/samd2x_l2x/family.cmake wires src/portable/microchip/samd/hcd_samd.c.
107 host firmware images were being compiled nowhere.

The merged family is wider than the old samd21 one, so three boards need an
explicit skip rather than the rename alone:

  atsaml21_xpro, saml22_feather, sensorwatch_m0 - not samd21, so hcd_samd.c is
    not compiled for them (previous commit); SAML22 has no host controller at all
  curiosity_nano - SAMD21 with 16 KB RAM; msc_file_explorer_freertos overflows it
    by 3688 bytes (ram 122.51%). Only that one example; the other eight fit.

Verified across the whole family: cmake 11 boards x 9/9 examples + curiosity_nano
8/9, three boards skipped, 0 failures; make 98 OK / 0 failed (was 98/9 before).
Device examples on saml21 and saml22 are unaffected.

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