dropbearkey: make rsa as a default value for -t #234

Job	Run time
build (plain linux)	41s
build (multi binary, 1, 1)	46s
build (multi binary, dropbearmulti argv0, 1, 1)	47s
build (client only, no, PROGRAMS=dbclient)	20s
build (server only, no, PROGRAMS=dropbear)	22s
build (bundled libtom, bionic , no writev(), ubuntu-20.04, --enable-bundled-libtom --enable-werro...	27s
build (linux clang, clang)	56s
build (c89, -std=c89 -Wdeclaration-after-statement)	42s
build (macos 12, macos-12, clang, -Wno-deprecated-declarations -Wno-undef, no, no, ranlib -no_war...	1m 2s
build (macos 11, macos-11, clang, -Wno-deprecated-declarations -Wno-undef, no, no, ranlib -no_war...	1m 4s
build (DEBUG_TRACE, #define DEBUG_TRACE 5 )	42s
build (nondefault options, 1, --enable-pam)	44s
build (most options disabled, --disable-harden --disable-zlib --disable-openpty --disable-lastlog...	21s
	8m 54s

Provide feedback