Support out-of-PATH installations

[stefanberger]

This PR implements a function find_program() that tries to find a program, which is only described by the program's name (e.g. 'swtpm_cert'), in BINDIR before trying $PATH and therefore will find swtpm_localca and swtpm_cert in non-PATH installation paths, such /home//rootfs/usr.

[coveralls]

Pull Request Test Coverage Report for Build 4285

• 11 of 11 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 73.324%

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Totals
Change from base Build 4274:	0.02%
Covered Lines:	7075
Relevant Lines:	9649

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💛 - Coveralls