net/ipxe: update 20200722 to g20201020,1 (e6f9054)

build OK
testport OK (supported archs)

while here,

• adopt port;
• follow "5.4.3. USE_GITHUB" recommendations; [1]
• PORTEPOCH needed to be used here;
• reduce GH_TAGNAME to use 7 (seven) chars as usual.

[1] freebsd.org/doc/en_US.ISO8859-1/books/porters-handbook/makefile-distfiles.html

PR: 250493