Let boot(8) refer to the uart(4) serial driver, which is the replacement for the obsolete sio(4) driver.
Let boot(8) refer to the uart(4) serial driver, which is the replacement
for the obsolete sio(4) driver.

PR:             docs/144498
Submitted by:   Bruce Cran (bruce at cran dot org dot uk)
Approved by:    marcel
MFC after:      2 weeks

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